Northern Prairie Wildlife Research Center
Relevant to Platte River system conservation is the ecosystem management approach being adopted by USFWS. USFWS (1994b) divided the U.S. into several ecosystems, including the Platte/Kansas ecosystem, to pursue an ecosystem management approach to USFWS's trust responsibilities for fish and wildlife resources. "Ecosystem approach" or "ecosystem management" is not a new concept. It was central to Aldo Leopold's eloquent discourses about conservation biology and the need for a "land ethic," and its importance to resource conservation was expressed by others during and prior to Leopold's day. Leopold initially viewed natural resources solely as a commodity, but through experience and observation, he shifted his focus to emphasize the landscape as a system of interrelated processes. In his later years, Leopold maintained that the most important goal of land management is to sustain the health of natural ecosystems and ecological processes. He believed this could be accomplished even when permitting human use of natural resources, provided such use was congruous with protecting biological diversity.
Drawing from the current literature and the many ongoing state, federal, and private efforts, an ecosystem approach for the Platte/Kansas ecosystem or any ecosystem can be generally characterized as follows (USFWS 1994b):
Recently, the National Biological Service (Noss et al. 1995), in its assessment of ecosystem losses throughout the U.S., concluded that an ecosystem approach would greatly enhance species-level conservation:
Ecosystem conservation is a complement to—not a substitute for—species-level conservation. Protecting and restoring ecosystems serve to protect species about which little is known and to provide the opportunity to protect species while they are still common. Yet, ecosystems remain less tangible than species (Noss 1991). And although the logic behind habitat protection as a means of conserving biodiversity is difficult to refute, conservationists face a major hurdle: convincing policy makers that significantly more and different kinds of habitat must be designated as reserves or otherwise managed for natural values. Scientists cannot yet say with accuracy how much land or what percentage of an ecosystem type must be kept in a natural condition to maintain viable populations of a given proportion of the native biota or the ecological processes of an ecosystem. However, few biologists doubt that the current level of protection is inadequate
A continually expanding list of endangered species seems inevitable unless trends of habitat destruction are reversed soon through a national commitment to ecosystem protection and restoration. A strategy for ecosystem conservation must not only protect and restore ecosystem types that are already endangered but must be proactive by conserving multiple, healthy examples of all native ecosystem types in each region (Tear et al. 1993).
Ecosystem conservation offers several advantages over a species-by-species approach for the protection of biodiversity: it directly addresses the primary cause of many species declines (habitat destruction), it offers a meaningful surrogate to surveying every species, and it provides a cost-effective means for simultaneous conservation and recovery of groups of species. The species-by-species approach—although extremely important to our efforts of saving biodiversity—is inefficient (LaRoe 1993). As the public becomes more familiar with the evidence that entire ecosystems or groups of species have declined and that saving individual species under the Endangered Species Act of 1973 does not solve all conservation problems and does not necessarily prevent the need for future listings, the rationale for ecosystem conservation becomes more compelling.
It is clear from the above and the appendices that the Platte River system still contains a wealth of biodiversity. Some species are federally and state listed as endangered or threatened because the ecosystem upon which they depend is deteriorating. Here we discuss past and present conditions along the Platte River, specifically the central Platte, in an attempt to view the ecosystem historically and provide a backdrop for possible management options. At the outset, we recognize that the former Platte River ecosystem cannot be re-established but affirm that the current ecosystem can be rescued and restored through a variety of management actions.
Most historical observations of the Platte River were made in the spring (May-June) during the high flow period, because travelers had to begin their journey in time to cross the Rocky Mountain passes before snowfall (Eschner et al. 1981). We found no accounts of overbank flooding on the Platte in the historical record, even though the river was normally bank-full in the springtime (James 1923, Morgan 1963). There were, however, no bridges or roads across the river channel that could be used as markers to judge overbank flows. The complex of wetlands adjacent to the river channel might also have masked the effects of overbank flows, because they held standing water for most of the year (Woodbury 1847).
Historically, flows in the Platte River were high in the spring and relatively low in mid-summer (Eschner et al. 1981). However, there seemed to always have been a subsurface flow because travelers were able to obtain water by sinking open-ended barrels into the channel and by digging shallow holes (Clarke 1902, Ghent 1929, Ware 1911). Miller (1978) contended that the Platte River rarely went dry prior to the development of irrigation. Miller's conclusion was based upon the hundreds of canals that were constructed along the Platte River and its tributaries between 1860 and 1890 to directly divert summer flows for irrigation. The canals would not have been constructed if there was no summer flow to divert. Irrigation influenced the flow of the river during the early settlement period. Nearly 2,000 canals were on the North Platte and South Platte rivers by 1890. Flows in the South Platte River were overappropriated between 1880 and 1885, whereas flows in much of the North Platte River were overappropriated by 1901 (Eschner et al. 1981).
Changes in channel width are the best available measure of historical channel geometry of the Platte River. Although data on channel depth, slope, and velocity are available from several gaging stations throughout their periods of record, most of those stations were located at or near bridges and do not represent channel conditions throughout most of the Big Bend reach (Lyons and Randle 1988). However, long term changes in bed elevation at bridges are good indicators of bed elevation changes in natural reaches.
Trends in channel sinuosity and a measure of channel braiding were presented in Williams (1978). A braided channel consists of numerous, interconnected small channels between shifting gravel bars and sandbars. Braided channels characterize streams with a large sediment load compared to meandering streams. Williams reported a more sinuous channel through the Big Bend reach, as measured from 1969 aerial photography, compared to 1938 photography, with two exceptions. On a short reach below the J-2 power plant return, the Platte River was straighter in 1969 than in 1938, and downstream of Gibbon to Grand Island, a 31-mile reach showed no change in sinuosity from 1938-1969. Williams' braiding index (the ratio of vegetated and unvegetated island length in a reach to the total reach length) showed a less braided channel from 1938-1969, except for the Overton to Grand Island reach where portions of the channel had become slightly more braided.
Changes in channel width for the Platte River in the Big Bend reach have been studied and reported by Williams (1978), Eschner et al. (1983), Peake et al. (1985), Becker (1986), Sidle et al. (1989), Johnson (1994), and others. Based on an interpretation of aerial photographs and maps, Peake et al. (1985) provided estimates of channel narrowing at six locations along the Platte River from 1865 through 1983 (Figure 4, Table11). The rate of channel narrowing increased at all six sites from 1938 to 1957, but decreased since then. The four upstream sites (Brady, Gothenburg, Cozad, and Overton) show little change in channel width from 1957 to 1983, whereas the channel at Grand Island and Odessa has continued to narrow. The channel narrowed from a mean of 2,707 to 1,339 feet during 1865-1983. In contrast, the mean channel width at Overton decreased from 4,795 to 1,050 feet over the same time interval; a decrease of nearly 78 percent. The mean channel width at Overton has remained relatively unchanged since 1957, showing only an 8 percent decrease from 1,139 to 1,050 feet. Johnson (1994) stated that "although Platte River channels have stabilized in width in recent decades, temporary narrowing will occur during droughts. More permanent narrowing could result if climate changed or exploitation of streamflow and ground water increased. Further channel narrowing may not benefit certain rare or uncommon migratory birds that utilize wide, active channels."
On the North Platte River in Nebraska, Williams (1978) found even greater losses of channel width than on the Platte River. In surveys during the 1860s, the average width of the North Platte channel (2,608 feet) was considerably less than the average width of the Platte River in the Overton to Grand Island reach (4,124 feet). Water development on the North Platte and Platte rivers also has been substantially different. For this reason, one expects differential changes in river channel morphology. Channel widths in 1965 ranged from 6 percent to 26 percent of the 1865 widths along the Lewellen to North Platte reach of the North Platte River. The greatest declines in channel width occurred between Sutherland and North Platte. Width declined less near Lewellen at the upper end of Lake McConaughy.
The declines in channel width described by Williams (1978) and Eschner et al. (1981) are indicative of the changes in channel morphology along the Platte and North Platte rivers, but do not provide a quantitative assessment of the acreage of habitat lost during development. A comparison of acreages in three river reaches along the Platte was made between years 1860, 1938, 1969, and 1982. Based on the work of Eschner et al. (1981) it was assumed that the river channel in 1860 was almost entirely without woody vegetation.
Channel Maintenance Flows
Instream flows to maintain the braided, unvegetated character of the river channel (channel maintenance flows) are needed to restore and maintain the ecosystem upon which sandhill cranes, waterfowl, whooping cranes, piping plovers, least terns, bald eagles, and other wildlife depend (Currier et al. 1985, USFWS 1987a). Much of the earlier discussion of habitat descriptions related to both long-term channel maintenance flows, and seasonal flows required during periods of species occupancy. Based on the historic occurrence of peak flows, the USFWS recommended a 5-day period of flows greater than 8,000 cfs to satisfy channel maintenance requirements (USFWS 1987a). Lyons and Randle (1988) considered the 1,000 to 10,000 cfs span of flows (using the 1958-1986 period of record at Overton and Grand Island) as the range of channel forming flows under current conditions of bed material supply. However, their review of aerial photography showed channel width at Overton decreasing slightly from 1958 to 1983 and the mean channel width at Grand Island also decreasing during the same period.
Much of the following was developed by the Platte River Management Joint Study (Joint Study) (Hydrology Work Group 1989). Channel morphology adjustments are determined by a complex series of independent variables and their interaction. The importance of each variable as it relates to channel shape may also vary with time. Variables influencing channel geometry include geology, paleoclimate, paleohydrology, relief, valley dimensions, climate, vegetation, land use, and hydrology. The first six variables are not readily influenced by human activity, but the remaining variables may be influenced. Although channel morphology reflects the influence of the above independent variables, the nature and quantity of sediment and water moving through a channel largely determines the morphology of stable alluvial channels.
Schumm (1977) developed an empirical relation showing that discharge and bed material load influence several channel measures, including bankfull width, meander wavelength, width/depth ratio, sinuosity, slope, and bankfull depth. For a reduction in both discharge and bed material load, such as happened in the Platte River, Schumm's relationship predicts that width, meander wavelength, and width/depth ratio should decrease while sinuosity should increase. The influence of decreasing discharge and bed material load on channel slope and depth are opposite in Schumm's relation, and it is not clear in what manner slope and depth should change. However, because the width/depth ratio decreases, the bankfull channel depth should remain constant or increase (deepen) and slope decrease due to increased sinuosity. The historic response of the Platte River to reductions in discharge and bed material supply has been consistent with Schumm's empirical relation, most notably an increase in sinuosity and a reduction in width and width/depth ratio.
Reductions in discharge, sediment supply, and bed degradation allow vegetation to initiate growth in the inactive river channel. The new vegetation stabilizes inactive areas and protects them from subsequent erosion. This cause and effect relationship may occur in a short period and may be permanent. Three to five years of reduced flow levels appear to be sufficient to permit vegetation to stabilize above the stages not scoured by subsequent peak flows.
Effective discharge is defined as the increment of sediment-transporting discharge that transports the largest fraction of the sediment load over a period of years. Effective discharge was computed for the Platte River by Lyons and Randle (1988) for three time periods: 1926-1939, 1940 to 1957 and 1958 to 1986. The effective discharges for each period are 3,900 cfs, 1,650 cfs, and 1,600 cfs, respectively. Compared to the earliest time period, the latter two periods do not have a distinguishing flow for which most of the sediment is transported.
Although Lyons and Randle (1988) considered the 1,000 to 10,000 cfs span of flows (1958-1986 period) as the range of channel forming flows, only 55 percent of the sand load is transported by flows in this range. Thus, a significant portion of the sand load is transported by flows exceeding 10,000 cfs and the frequency of these flows is also critical in maintaining the existing channel dimensions. For example, the span of flows which transported about 85 percent of the sand load during the same period is 1,000 to 19,000 cfs.
Although effective discharge cannot be computed for the earliest periods without adequate sediment rating curves, the magnitude of alteration in effective discharge and reduction in sand load transported can be illustrated with the reduction in mean annual peak flows. Before development, the central Platte River had a wide, shallow bed with numerous transient bars and mean annual flows greater than 25,000 cfs. Construction of six reservoirs on the North Platte River during 1909-1957 contributed to a successive decrease in peak flows and sediment supply. From 1895 to 1909, peak discharges averaged 19,270 cfs. From 1910 to 1927, mean peak discharge was 11,050 cfs. The mean was reduced to 8,710 cfs from 1928 to 1941, and to 3,330 cfs from 1942 to 1980 (U.S. Geological Survey, unpublished data).
Lake McConaughy is the most recent downstream barrier to sediment sizes which are found in significant percentages in the Platte River bed. Historically, the North Platte River contributed at least 60 percent of the sand load at Overton. The estimated sand load at Overton for the 1926-1939 period was 2.1 million tons/year, and 603,000 tons/year for the more recent 1953 to 1985 period. Present day sand loads at Overton are 30 percent of the estimated historical values (Lyons and Randle 1988). The river has responded to the reduction in sediment supply by degrading 3 to 5 feet from Lexington to Chapman (U.S. Bureau of Reclamation, unpublished data).
Although the gage record at Julesburg, Colorado, does not precede diversion of South Platte River water, it appears that peak flows may have been influenced by diversions in the absence of mainstem reservoir impoundment. Prior to 1924, the South Platte peak flows at Julesburg averaged 7,400 cfs. At present, the mean annual peak flow is 4,900 cfs, a 33 percent reduction caused by upstream diversion. The South Platte River is now the dominant tributary for sediment input into the Platte River.
An analysis of tree rings from the river floodplain (Bunde et al. 1975, Currier 1982) suggests that most of the trees lining the bank were established from 1935 to 1960 following a reduction in peak flows and an increase in the length of time between peak flows. This period corresponds with the large reduction in sediment transport observed in the Platte River.
There may be a correlation between the decreasing number of no-flow days in recent years (1940-present) and decreasing channel width. However, the flows during the germination period (late summer and early fall) during the pre-development period (pre-1909) were higher than in post development years. From 1918 to 1929, there were 18 no-flow days per year. During 1930 to 1941, there was an average of 75 no-flow days annually. After 1941, there were no periods of zero flow until 1988. Zero flow days in the 1930s resulted from extended drought and upstream diversions. Channel width at Overton decreased markedly during the low-flow periods of the 1930s (Williams 1978).
Seasonal Habitat Flows for Endangered Species
Channel maintenance flows are vital to the continued presence of an unvegetated or lightly vegetated stream channel. This criterion alone is not sufficient to assure the continued occupancy of riverine habitats by endangered species, or other habitat-sensitive fauna. Based on field and empirical data and mathematical models, the USFWS developed a recommended flow regime for the central Platte River designed to satisfy the seasonal requirements of endangered birds and their forage fishes. The time periods were selected based on occupancy periods for each resource of concern.
Faanes and Bowman (1992) reported that flows of 838 to 5,150 cfs existed in the Platte River on 16 dates when whooping cranes occupied the Big Bend reach of the Platte River. Ninety percent of the whooping crane use occurred when flows were greater than 1,200 cfs. The mean flow during all confirmed sightings of whooping cranes was 2,683 cfs, fully 55 percent greater than the 1,200 cfs minimum. Faanes and Bowman (1992) recommended 2,000 cfs as the minimum roosting habitat flow for whooping cranes based on (1) existing habitat conditions, (2) present population levels of the whooping crane, (3) knowledge of the species' migrational habitat requirements, and (4) knowledge of the effects of flows and flow changes on suitability and availability of whooping crane roost habitat. A recent roosting habitat modeling effort (USFWS, unpubl. data) suggests that 2,400 - 2,500 cfs may provide the highest quality habitat flows under existing conditions.
Wet meadows are an important component of Platte River habitat. Sufficient flows are necessary to initiate biological productivity in wet meadow vegetation and invertebrate populations used by migrating whooping cranes and sandhill cranes in spring. Flows are occasionally less than 1,100 cfs during February to early May. Wet meadows should be maintained if a flow of 1,100 cfs is provided during that time period.
The response of wet meadows to changing hydrologic conditions is still not well understood (Henszey and Wesche 1993). Plant species and presumably wet meadow fauna respond dramatically to fluctuations in wet meadow hydrology (Currier 1989). Wet meadows close to the Platte River are most likely to survive because they are hydraulically connected to the river and may be more influenced by river stage. Wet meadows farther from the river are more likely to be influenced by declines in ground water levels.
Both least terns and piping plovers have specific minimum and maximum daily flow requirements. Recommended flows are needed during the nesting season to deter access by terrestrial predators and to bar disruptive human incursions to nesting colonies (e.g., all-terrain vehicle traffic), Recommended maximum flows are essential in reducing the likelihood that high flows may inundate suitable riverine nest sites during both nest initiation and active nesting periods.
Faanes (1983) observed that nesting areas on the central Platte River were inundated on June 21, 1979. U.S. Geological Survey records indicate that flows rose from 1,810 cfs on June 20 to 3,000 cfs on June 21. Flows were less than 1,000 cfs during nest initiation. Elsewhere, nests were initiated on a sandbar south of Alda, Nebraska, on July 10-12, 1984 (G.R. Lingle, unpubl. data). U.S. Geological Survey data indicate that previously high flows decreased from 8,000 cfs just prior to nest initiation, and ranged from 3,500 cfs on July 10 to 2,370 cfs on July 12. Ziewitz et al. (1992) found least tern and piping plover nests at innundation flows of between 200 and 5,600 cfs with a median of 2,600 cfs. They concluded that very little riverine nesting habitat remains on the central Platte. Indeed, most nesting occurs at sand and gravel pits (Sidle and Kirsch 1993).
Most piping plovers and least terns nest on sandbars at elevations equivalent to flows less than 2,500 cfs (Ziewitz et al. 1992). Maximum daily flows of 2,500 cfs should not be exceeded in the central Platte River during the principal nesting period from June 1 to August 25. Conversely, flows should not drop below 800 cfs at the same time to ensure an adequate barrier to predators and all-terrain vehicles.
Least terns forage for small fishes while nesting in the Platte River valley. Observations revealed that sandpits are not used frequently as foraging sites (NGPC 1985, G. R. Lingle, Platte River Whooping Crane Habitat Maintenance Trust, Inc. (Trust), unpubl. data). Flows that provide habitat for the survival and annual production of forage fishes used by least terns must be maintained all year. Minimum flows must be adequate to provide fish habitat and sustain adequate water quality and limnological functions of a lotic system. Flow-related concerns in modeling fish habitat included water quality, temperature, and substrate composition (Fannin and Nelson 1986).
Temperature extremes can be stressful or lethal to fish populations (Dinan 1992, Lantz 1970, Fry 1971, Matthews et al. 1982). Low flow or sudden changes in flow accompanied by changes in physical, chemical, or biological factors, can create poor water quality conditions adversely affecting fish populations (Fry 1971, Theurer et al. 1984). Impacts to fish populations can occur in certain circumstances related to reduced flow especially in summer. A fish kill occurred in June 1991 when flows were between 400 and 800 cfs. A minimum flow of 800 cfs is necessary to ensure fish habitats during June 1 to September 15.
Bald eagles can be found anywhere in the Big Bend of the Platte River during mid-October to early December. Weather conditions are typically mild and the river is generally ice-free. Beginning in late December, the weather becomes more harsh and the river freezes. Most bald eagles then move to a reach of ice-free river downstream from the J-2 river return or other areas kept open by warm ground water springs. Flows in this area are usually of sufficient quantity to maintain open water for about 8 miles downstream. With normal or below normal temperatures during December 10 to February 25, this 8-mile reach provides the only suitable bald eagle wintering habitat for at least 40 miles upstream or downstream. We believe that a flow of more than 1,100 cfs in this area during December 10 to February 25 will ensure that open water is available for foraging bald eagles.
Conservation of the Platte River system recognizes the need for instream flows for various ecosystem purposes including endangered and threatened species conservation (Platte River Management Joint Study 1990, 1993). Required flows vary widely because the needs of the ecosystem are varied. High flows are needed to scour developing vegetation and minimum flows are needed to protect fish life. Some flows in some years may not provide the best habitat for a certain species in that year, but in the long term are better for that species and the ecosystem. An example might be a high spring flow which impairs nesting by least terns and piping plovers but which improves their habitat in succeeding years (Sidle et al. 1992). Johnson (1994) recognized the need for managed flows to maintain vegetative growth dynamics. He suggested several options:
(1) prohibit recruitment in the active channel by augmenting June flows to maintain a several-year average of at least 75-85 m3/s (2,648-3,000 cfs) below the J-2 return and 30-40 m3/s (1,059-1,412 cfs) above the Return; and increase seedling mortality by (2) raising winter flows to increase ice scouring, (3) increasing spring peak erosive flows to remove seedlings, or (4) reducing late-summer flows to increase seedling destruction.
Options 1 and 3 would require large flow releases from storage during dry years to be effective. Option 3 would require flows of at least 170-225 m3/s (6,000-7,946 cfs) for several days to remove or bury 1st-year seedlings. Removal of older seedlings would require considerably higher flows, and established woodland could only be removed on islands and banks by floods of large magnitude, comparable to those of 700 m3/s (24,720 cfs) or so that occurred in 1983.
Johnson (1994) recommended a combination of the above flows and experimentation and monitoring to gage the results of various flows.
Bowman (1994) and Bowman and Carlson (1994) summarize the USFWS's recommended flows for the central Platte River ecosystem. The flows are to achieve the USFWS's goal of rehabilitating and maintaining the structure and function, patterns and processes, and habitat of the central Platte River valley ecosystem. The flows are designed to complement landscape rehabilitation for listed species, comprising approximately 29,000 acres in ten segments between Lexington and Chapman, Nebraska (Platte River Management Joint Study 1990 and 1993).
The ecosystem-oriented approach includes the objectives of (a) recovering habitats of presently listed species, (b) preventing the need for listing additional species, and (c) providing sufficient habitat for conservation of native biotic components of the ecosystem. This goal corresponds with the USFWS's policy of conservation management at the ecosystem level and with purposes stated in section 2(b) of the ESA, as amended: "...to provide a means whereby the ecosystems upon which endangered species and threatened species depend may be conserved, to provide a program for the conservation of such endangered species and threatened species, and to take such steps as may be appropriate to achieve the purposes of the treaties and conventions set forth in subsection (a) of this section."
USFWS recommended flows are based upon data and models described and referenced in various regulatory correspondence to the Federal Energy Regulatory Commission (FERC) and the U.S. Forest Service and incorporate several assumptions:
Four categories of stream flows are identified: seasonal pulse or peak flows; seasonal flows characteristic of wet years; flows characteristic of normal or average years; and flows characteristic of dry years. The annual volume of the target flows for wet, average, and dry conditions is 1,117,900 acre-feet, 1,063,100 acre-feet, and 715,400 acre-feet, respectively (Table 12).
Dry year flows were framed by using biological criteria. Dry year flows particularly limit the survival and life cycles of aquatic and wetland species, which are the species affected acutely by low flows. The fish community is the dry year target community because it is representative of aquatic species in the ecosystem and some fish species have life cycles of three years or less. Therefore, dry year flows should not occur on the average more often than once every four years.
Normal year flows are neither dry year nor wet year flows and occur or are exceeded on an average basis at a frequency of three out of four years. Normal year flows provide some habitat for all communities in the ecosystem during all the seasons (time periods). Normal flows provide habitat for and sustain populations of most species in the ecosystem between episodes of dry and wet year flows. Extreme flow events (i.e., variations in magnitude, timing, and frequency of-flows, in normal years should not be diminished).
Rule triggers for determining whether a year is likely to fall in the category of wet, normal, or dry and for making water resource management decisions for each year type should be based on estimates of the present gross water supply plus estimates of independent measures of water supply, such as ground water, precipitation, and snowpack, comprising the gross water supply in the entire Platte River Basin. Rule triggers and flow management decisions based only on dependent variables such as reservoir storage, project-by-project capabilities, or projections of water availability from water projects likely would lead to water management decisions that reflect only dry year conditions and little operating flexibility.
May and June Pulse Flows:
February and March Pulse Flows: Wet year priority = 2
Pulse flows which mimic the natural hydrograph are needed to restore, on a reduced scale, certain annual effects characteristic of the historic natural hydrograph. These natural surges in flows have been severely depleted since the pre-development era. Pulse flows are necessary for sediment transport, for redistribution and deposition of sediment in the central Platte River, and for shaping channel morphology into wide, shallow channels. Pulse flows generate a diversity of habitats across the floodplain; drive ecosystem processes in backwaters and wet meadows such as thawing and stimulation of biological activity that ultimately produces food for animals and favorable habitat for both animals and plants, including threatened and endangered species. Timing of pulse flows coincide with or influence fish reproductive behavior and the availability and quality of spawning, nursery, and rearing habitat, including backwater habitat of fish and mollusks. Flow pulses, especially those which move ice and sediment, scour vegetation of different size and age classes and prevent reestablishment of vegetation. (see following section, Pulse Flows).
May 11-September 15: Wet year priority = 3 Normal year priority = 1 Dry year priority = 1
This period is when most species in the ecosystem face their most critical water shortages. Therefore, proportionately greater biological stress and ecological effects can occur if water is withdrawn or withheld from the ecosystem during this period. Maintaining the components of biological diversity, e.g., plants, invertebrates, fishes, and birds, during this period depends on the aquatic component of the ecosystem. Flows are needed to provide essential habitat components for threatened and endangered species, as well as other important native wildlife populations.
This period is when aquatic shorebirds, such as the piping plover and least tern, are mating, nesting, and rearing young. Target flows for this period, particularly May 11 to June 15, help prevent shorebirds from nesting at such low elevations in the river channel that their nests would be subject to flooding during subsequent intervals of higher flows caused by local rainfall and/or flow regulation practices. Instream flows provide a degree of barrier to terrestrial predators which would otherwise more easily prey on shore bird nests. During summer, instream flow targets prevent losses from the native fish community by curtailing rises in water temperatures to levels that otherwise would be detrimental or lethal to a variety of life history stages of aquatic organisms, including fishes. The native fish community is a critical component in the ecosystem which has been harmed repeatedly by episodes of low flow during this time period in past years. The flow target for this period will prevent or reduce future harmful episodes to the aquatic community.
March 23-May 10: Wet year priority = 4 Normal year priority = 2 Dry year priority = 2
Except for the earliest migrating geese, this period is the primary spring migration period for birds through this region. Flows contribute important nutritional and physiological conditions for birds preparing to breed. For example, wet meadows are undergoing primary production of invertebrates which are needed by cranes for protein and calcium. Whooping crane migration habitat has been severely degraded as a result of decreased flows and loss of night roosting habitat critical at this time. Flows during this period also provide sandhill crane habitat. This is the time of year when Eskimo curlews are most likely to use the Platte River. Flows during this period provide channel habitat for water-dependent organisms, including spawning fish, mussels, and migratory waterfowl, wading birds, and shorebirds. Environmental education and eco-tourism (e.g., crane watching) are very important public and economic values during this time (Lingle 1991).
February 1-March 22: Wet year priority = 5 Normal year priority = 3 Dry year priority = 3
This is the second most important period for migratory birds during the spring. Bald eagles forage in the river valley during this period. Flows provide migrating waterfowl and other bird species with suitable migration habitat. They also provide sandhill cranes with suitable roosting sites and feeding habitat in wet meadows. Water in the Platte River valley ecosystem is of particular importance for early migrating waterfowl when Rainwater Basin wetlands are frozen, because it helps to disperse birds and reduce losses due to disease (e.g., avian cholera, botulism, etc.). Flows in this period also form and move ice, which scours vegetation and shapes the channel. Fish habitat also is provided by these flows. This period does not have a higher priority because suitable flows are often met with present conditions. However, other comparable springtime habitats have been eliminated or are rare, such as Platte River and North Platte River channel and wet meadow habitats west of Overton.
September 16-30: Wet year priority = 6 Normal year priority = 4 Dry year priority = 6
These flows will maintain and prevent loss of the native fish community and will promote survival of fish young-of-year.
October 1-November 15: Wet year priority = 7 Normal year priority = 5 Dry year priority = 6
Flows during this time period provide migration habitat for migrating waterfowl and other migratory bird species (e.g., fall whooping crane migration and roosting habitat). These flows also maintain aquatic life; for example, they promote growth of young-of-year fish. This period may have been a moderate or low flow period naturally and that whooping crane sighting data indicate that whoopers use the river less in fall than in spring. Perhaps the normal and wet year targets could be the same as the present-day dry year target. However, flows in this period support waterfowl habitat and recreational activities, such as waterfowl hunting, that are important public values.
November 16-December 31: Wet year priority = 8 Normal year priority = 6 Dry year priority = 5
Flows during this period provide bald eagle feeding habitat. These flows also maintain fish habitats necessary to support fish communities. The use of the Platte River by migratory birds and geese also was considered when prioritizing this time period. Goose hunting is an important public activity during this time period.
January 1-31: Wet year priority = 9 Normal year priority = 7 Dry year priority = 4
Flows in this period provide foraging habitat for bald eagles and other raptors. Viewing of foraging bald eagles provides a public recreational benefit during winter conditions. January flows also promote the winter survival of the native fish community and aquatic insects. The flows form and move ice to scour vegetation and maintain the channel. Although it is recognized that base flows are important during this period, it is not ranked higher because flows are frequently adequate with present operations. Dry year target flows during this period could be inadequate to sustain fish if severely cold weather occurred concurrently and froze the river to the extent that fish habitat deteriorated to the point of limiting fish survival.
The USFWS believes that various flows are needed for species recovery (Table 13) but that pulse flows in late spring and late winter (Table 14 and Table 15) are the highest and second highest priorities, respectively, for achieving ecosystem goals.
Pulse flows occur at some magnitude and duration in wet, normal, and dry years. During normal and wet years, pulse flows inundate wet meadows, increase hydrophytic vegetation, scour vegetation, prevent nesting by shore birds at low elevations on sandbars, inundate backwater areas, form sandbars, and form and/or move ice. To maximize their effectiveness, pulse flows must be of sufficient timing, magnitude, and duration to scour seedlings off sandbars and prevent seed germination, as well as to trigger the response of the aquatic community, e.g., spawning fish. Pulse flows play a dominant role in the patterns and processes, structure and function, and habitat of the Platte River valley ecosystem. The magnitude of pulse flows and overall annual flows have changed since the turn of the century (Figure 3 and Figure 5).
The magnitude and duration of pulse flows include an average of 8,000 cfs for five days in June for channel maintenance; an average of 3,800 cfs during 61 days in May and June, an average of 5,800 cfs for 30 days during May and June, an average of 3,200 cfs during 60 days in February and March; and an average of 4,400 cfs during 30 days in February and March. Sandbars were formed in 1983-1984 at flows of about 20,000 cfs. Flows of 2,600-3,000 cfs in June prevents germination of tree seeds. Flows of 6,000-8,000 cfs in February and March removes seedling vegetation. Approximately 23 percent of the time, flows in February and March are 2,950-3,700 cfs. The frequency, magnitude, and duration of extreme flow events which occur as variations in flows during February-March and May-June of normal and wet years should not be reduced.
Pulse flows should occur with their natural timing, during late winter and late spring. For these periods, conditions for wet, normal, and dry hydrologic conditions were adapted (Bowman 1994). A fourth condition called "very wet" represents those years in which peak runoff is very high, and results in surface flow in wet meadows, side channels, sloughs, and backwater areas. Occurrence of this condition is necessary to maintain and enhance the diversity, distribution, and abundance of habitats and organisms in the Platte River valley ecosystem.
The importance of sediment movement and availability in forming and maintaining the geomorphology of the Platte River channel is well recognized. The rates of channel narrowing decreased significantly during approximately 1969-1986, though some further narrowing may have occurred since that time. Whether the Platte River channel is in equilibrium, quasi-equilibrium, or will continue to narrow is still debated.
The 1969-1986 period was selected by USFWS as defining minimum conditions (i.e., frequency and magnitude) of peak flows which should be retained and increased primarily for the five-year and more frequent events. The recommended objective is for a ten-year running average of mean annual peak flows ranging from approximately 8,300 cfs to 10,800 cfs; this objective should be achieved through adaptive management of water resources if natural events are not sufficient to do so. This range is based on an average of channel maintenance properties computed for the Platte River with five different approaches. The mean annual peak at Grand Island during 1969-1986 was 9,124 cfs.
The largest pulse flow events (i.e., > 12,000 cfs) will be natural occurrences beyond the control of water resource managers in the Platte River Basin. The pulse flow targets described herein do not imply that USFWS recommends flooding along the Platte River. However, the capacity of some channel sections of the North Platte and the Platte rivers have become reduced, yet high flows are still necessary to maintain channel capacity. USFWS intends to work with other agencies and local interests to maintain and improve channel capacity. Public and private works projects designed to increase channel capacity through removal of woody vegetation should be encouraged. Such actions not only would reduce the likelihood of out-of-bank flooding during uncontrolled high flow events while increasing the availability of sediment but would increase and/or enhance channel habitat for waterfowl, cranes, and other migratory birds; reduce the need for bank stabilization projects; and increase and/or enhance opportunities for recreation in the channel. Specific management may be needed to protect the armor layer in the North Platte River channel below Kingsley Dam to prevent removal by scouring flows.
Recruitment of cottonwoods should be managed by the magnitude of pulse flows rather than by continuous inundation of the active channel during the period of seed deposition and viability. Various factors contribute to seedling mortality. For purposes of seedling removal, the optimal time at which the late winter pulse flows in Table 16 should occur is during ice break-up.
River stage is most frequently the dominant influence on ground water levels in wet meadows, and composition and structure of biological communities in grassland is most closely associated with the environmental variable of soil moisture. Pulse timing should correspond with naturally occurring periods of high runoff, and hence physical processes and critical life stages of aquatic and semi-aquatic biota. During the growing season, a duration of 7-30 consecutive days provides minimal wetland hydrology (e.g., anaerobic conditions supporting hydrophytic plants). Life stages of some aquatic and semi-aquatic wet meadow organisms require up to 30 days, and possibly longer. Some meadows are wet in a pattern similar to current flow events (i.e., the 1969-1986 flow records). Some wet meadows have elevated ground water, and added pulse flows would rehabilitate a number of these potentially "active" wet meadows in the ecosystem.
The recommended objective during May/June is a 30-day exceedence level having a ten-year running average (the flow met or exceeded for 30 consecutive days each year, averaged over a ten-year period) of at least 3,400 cfs. The 30-day exceedence level should vary year to year. As during 1969-1986, 3,000 cfs should be exceeded for 7-30 consecutive days in at least 75 percent of the years. Pulse flows should be followed by a descending rate not exceeding 800 cfs/day. No pulse flow is required in May/June in 25 percent of the years; base flows identified for species apply instead.
Pulse flow targets for the late spring period of May and June are necessary to provide the following effects in the ecosystem:
Pulse flow targets for the late winter period of February and March are necessary to provide the following kinds of beneficial effects in the ecosystem:
In years with little or no ice formation, pulse flows are necessary for soil saturation in meadows.
Given the central Platte's ecosystem water needs, water availability and shortages can be estimated as a backdrop for management decisions. The annual volume of the target flows for wet, average, and dry conditions is 1,117,900 acre-feet, 1,063,100 acre-feet, and 715,400 acre-feet, respectively (Table 12). These annual volumes do not include less frequent flow recommendations such as the 5-year peak flow of 16,000 cfs. For comparison purposes, the mean annual flow at Grand Island (1942-1993) is about 1,131,000 acre-feet, and the median of annual mean flows is about 854,000 acre-feet (Figure 6). The desired future occurrence of "wet" conditions is one out of every three years or 33 percent of the time. Dry conditions could occur once every four years or 25 percent of the time. Average conditions would therefore occur about 42 percent of the time.
There are three estimates of instream flow shortages for the flows desired on a relatively frequent basis (M. Butler, USFWS, personal communication). The estimates are computed using 1) mean annual recommendations and flows, 2) monthly recommended volumes and flows, and 3) daily recommended flows.
Estimate 1: Mean Annual Recommendations and Flows
I. Mean Annual Flows and Frequencies for Base Instream Habitat TargetsThe above is an underestimate of the instream flow shortage because the estimate assumes no difficulty in re-regulating flows in the Platte River basin.
Year Frequency Annual Volume wet 33% 1,117,000 acre-feet normal 42% 1,063,000 acre-feet dry 25% 715,400 acre-feet Weighted 994,400 acre-feet
II. February/March Pulse Flows Build Upon the Above Flows
February/March base instream habitat flow = 1,650 cfs (weighted mean) 30-day pulse flow = 3,350 cfs (75%) = 2,250 cfs (25%) weighted = 3,075 cfs 3,075 cfs -1,650 cfs = additional 1,425 cfs for 30 days = additional 84,800 acre-feet in Feb/Mar
May/June base instream habitat flow = 1,100 cfs (weighted mean) (10-year) May/June pulse target = 3,400 cfs for 30 days + descending limb @ 800 cfs/day 3,400 cfs - 1,100 cfs = additional 2,300 cfs for 30 days (136,800 acre-feet)(+ descent @ 800 cfs/day (6,800 acre-feet) = average additional flow of 143,600 acre-feet in May/June
IV. Mean Peak
A mean (10-year) annual peak of 9,550 cfs (5 days with ascending and descending limbs) is targeted during February-June. By building upon the 30-day pulse duration for February/March and May/June, the added volume of the peak is estimated:
9,550 cfs - 3,400 cfs = 6,150 cfs (5 days) + ascending @ 950 cfs/day and descending @ 795 cfs/day = 146,300 acre-feet (peak)
V. Estimated annual flow target = 994,400+84,800 + 143,600 + 146,300 = 1,369,100 acre-feet Grand Island annual flow (1942-1993) = 1,131,000 acre-feet Grand Island mean annual flow (1942-1993)(Opstudy* model present conditions) = 1,136,000 acre-feet Shortage Estimate (flow target-mean annual flow = mean annual shortage) (1,369,100-1,131,000 = 238,000 acre-feet) (1,369,100-1,136,000 = 233,000 acre-feet)
Estimate 2. Monthly Recommendations and Flows
Table 16 shows the wet, average, and dry years based on mean annual flow at Grand Island. Based on historical daily flows at Grand Island (1943-1992), the mean annual shortage is approximately 348,000 acre-feet for the combined wet/normal years and approximately 294,000 acre-feet during dry years (Table 17).
Annualized pulse flows result in a 200,000 acre-feet shortage relative to the 456,000 acre-feet need (Table 18, Figure 7). This is approximately double the shortage relative to the 3,000 cfs target for the May and June period. However, this calculation overestimates the May-June pulse flow shortage because it only determines shortages and does not give consideration to peak flow events which exceed approximately 10,000 cfs and bear on any long-term average.
Estimate 3. Daily Recommendations and Flows
Shortages for the combined wet/average and dry years are 417,000 acre-feet and 333,000 acre-feet, respectively, when each daily flow at the Grand Island gage is compared to the daily flow target in effect (Table 19). The weighted average is 396,000 acre-feet. This method probably overestimates the shortage because it does not account for potential changes in timing.
Implementation of the USFWS target flows would require reduced consumption of about 238,000 acre-feet (Figures 8 and 9) and 189,000 acre-feet from the re-regulation of existing water projects. The U.S. Bureau of Reclamation (1992) estimated consumptive use and the crop irrigation requirements for 11 stream reaches above Grand Island. The "consumptive use" estimate of 3.65 MAF represents total water needs of the various agricultural crops, including that portion supplied by precipitation. The "crop irrigation requirement" estimate of 1.66 MAF represents the volume of water consumed by the crop, exclusive of precipitation, stored soil moisture, and groundwater, that is required consumptively for crop production. It does not include any conveyance loss or deep percolation loss associated with canal diversion and farm efficiency. Therefore, the crop irrigation requirement may be an under-estimate of demand on surface water supplies.
The municipal consumptive use is based on an average of 0.125 acre-feet per capita and a basin population of 2,978,100, for a total of approximately .37 million acre-feet. The estimated evaporation for principal reservoirs and regulated lakes, other lakes greater than 1.5 acres, and small ponds and reservoirs was .37, .034, and .30 million acre-feet, respectively. Only one-third of the principal streams and canals estimated evaporation was included to represent the portion evaporated by canals.
The total of the various consumptions is: Crop Irrigation Requirement........................1,656,153 acre-feet Municipal Consumptive Use............................372,262 acre-feet Evaporation Principal Reservoirs and Regulated Lakes.........369,870 acre-feet Other Lakes Greater Than 1.5 Acres................33,720 acre-feet Principal Canals................................ 124,663 acre-feet Small Ponds and Reservoirs.......................300,900 acre-feet TOTAL..............................................2,857,568 acre-feet
The available stream flow record at the Overton gage provides an estimate of 2.4 million acre-feet as the change in mean annual flow during the major period of surface water resource development in the Platte River basin (Figure 5). Although the level of water resource development can be quantified, it is difficult to separate out the conflicting influences of development and climatic cycles such as drought and wet periods on the flow record. Because the change in volume estimate does not incorporate any change in the timing of snowmelt runoff peaks due to storage effects throughout the basin, it can be argued that the total effects are not addressed.
The out-of-stream consumptive water use values developed by the Bureau of Reclamation provides an estimate of nearly 2.9 million acre-feet of consumptive use from surface water supplies in the Platte River basin. The estimate does not identify volumes of water lost from surface supplies due to canal seepage, farm efficiency, interaction with groundwater development, and losses to groundwater which ultimately leave the basin. Therefore, even if the consumptive use estimate is exactly correct, it under-estimates the total volume of water lost from the system.
In general, the first method estimated the change in surface water supply, while the second method estimated the demand on surface water supply. Because both methods are only estimates, and one is no better than the other, the estimates are averaged together to provide a value of 2.65 million acre-feet. For reasons discussed above, the estimate of 2.65 million acre-feet lost from the Platte River basin due to current levels of surface water resource development is probably conservative. However, it is the best available information at this time. The limitations and assumptions should be kept in mind when using the estimate.
Currier (1982) and Currier et al. (1985) described the process of wooded vegetation establishment along the central Platte River. Most wooded plant species presently occurring in the system were present in historic times (Fremont 1845), but extensive forested stands were scarce. Exotic species such as Russian-olive have made extensive advances along the Platte River system replacing native woody species (Knopf and Olson 1984, Knopf 1986, Olson and Knopf 1986). In 50 years most cottonwooods will be replaced by green ash and the projected climax species, Russian-olive (Currier 1982).
The Platte River floodplain was originally a complex of wet bottomlands (Currier et al. 1985). Scouring flows in spring inhibited the establishment of permanent stands of wooded vegetation along the Platte River (Currier et al. 1985). High flows were responsible for moving large quantities of sediment that aided in the removal of shallowly rooted seedlings. Such conditions are now rare on the central Platte and on most of the South Platte and North Platte river systems. The diminished hydrologic connectivity between meadows and rivers, intensive grazing, and fire restriction contribute to the expansion of native and exotic woody vegetation on grasslands and wet meadows.
Establishment of permanent wooded vegetation has created corridors of riparian vegetation suitable for the dispersal of fauna further west of historic ranges. Knopf (1986, 1992, 1994) discuss the extensive avifaunal mixing along the now heavily forested South Platte River. Eighty-two percent of all avian species in northeastern Colorado ocuur in riparian vegetation (Knopf 1985). Knopf (1986) stated that it is probable that 90 percent of the contemporary riparian avifauna on the western Great Plains was not present at the turn of the century.
Currier et al. (1985) analyzed the relation of habitat change (primarily forest development) to 120 bird species nesting along the central Platte. Habitat changes were considered beneficial for 53 percent of the species, detrimental to 31 percent, and of no consequence to 16 percent of the species. Generally, benefitting species were forest inhabitants whose populations and ranges expanded with increased forest area. Habitat sensitive species, including birds dependent on native grasslands and wet meadows, experienced habitat loss when their preferred habitats developed into forests. Wide-ranging species ubiquitous in their selection of habitats were generally unaffected by habitat change.
Manipulation of riparian forest communities represents one of the most difficult challenges for the future management of Platte River habitats. The principal objective for much of the Platte River must necessarily involve the provision of adequate habitat area for the endangered fauna and flora. To do so requires the removal of scattered areas of forested habitats and their replacement with managed sandbars and reclaimed wet meadows. Avian species experiencing the greatest declines in the Great Plains are grassland birds (Knopf 1988, Peterjohn 1995). Therefore, the conservation of grassland habitats, including wet meadows, should be a priority. Again, historical accounts support a grassland-bordered river system maintained by fire and bison grazing. It is probably not practical to remove every tree along the Platte River system and, if current riparian avifauna are not detrimental, resources should be placed into management efforts to conserve all remaining grasslands. Moreover, human uses and interests in wooded riparian areas are many and varied and would not support the removal of all trees (Knopf 1986, Knopf et al. 1988, Knopf 1992).
In the lower Platte River below Chapman, Nebraska, extensive scouring of vegetation still occurs, however, forest on the high banks are extensive due to fire restriction and agricultural practices. Only situations where vegetation clearing on islands is necessary to provide habitat for nesting least terns and piping plovers should be considered. Climax deciduous vegetation will allow for the invasion of area sensitive species requiring forest interior vegetation, while providing management options to enhance endangered resources.
Forest establishment has been most extensive in the Big Bend reach upstream to North Platte. This area is also used by most of the endangered species. Because most of the endangered species in the system occupy habitats in the Big Bend reach, we believe that vegetation manipulation should be most intensive between Chapman and Lexington, Nebraska. Management should focus on forested islands as well as grasslands which are developing into forests (wet meadows with developing cedar and Russian-olive). It is more prudent to invest resources into a lightly treed grassland that has some recovery potential, than into the problematical conversion of a dense forest block into a grassland/wet meadow. Highest priority areas for protection of large forest fragments should be centered on known nocturnal roost sites used by wintering bald eagles, and sites supporting nests of colonial waterbirds.
To facilitate management of island habitats, we recommend that up to 50 percent of the existing wooded islands remain in each of the existing bridge segments of the Big Bend reach. This allows additional management alternatives for sandbar-specific species (e.g., piping plovers and least terns) and recognizes the needs of invading species that comprise most of the wooded island community (e.g., willow flycatcher and Bell's vireo) (Faanes and Lingle, unpubl. data).
Virtually the entire river reach from Lexington upstream to North Platte, Nebraska, is encroached with wooded vegetation. This reach also presents an opportunity to exercise the most creative management schemes. To enhance sandbars for least terns and piping plovers, we recommend the same management as for the Big Bend reach. This requires vegetation removal from 50 percent of the islands in each bridge segment. Sandbar management here will be successful only with provision of adequate instream flows for protection of nesting colonies and a certain food supply. Sand and gravel companies could play a role in clearing the river channel.
Breeding bird population data suggest that the avifauna of the Lexington to North Platte river reach is the most highly developed of any area on the mainstem of the central Platte River. To retain diversity in this reach, we recommend the clearing of vegetation away from the active channel to maintain some forested tracts in each bridge segment.
We recommend management for the North Platte River reach between the city of North Platte and the Kingsley Dam, that resembles the recommended plan for the Big Bend reach. Because of the vital importance of this reach to nearly 100,000 sandhill cranes (Folk and Tacha 1990, 1991; USFWS 1981) and wintering and migrating waterfowl, we believe management should focus on those bird species whenever possible.
Before settlement, most of the grasslands in the Platte River valley were wetlands of a mixture of poorly drained sedge meadows and marshes and relatively well drained, slightly elevated lowland prairies with subsurface irrigation (Currier et al. 1985). The lowland prairies, which were characterized by big bluestem, were the dominant feature in the valley. Short and mid-grass prairies of little bluestem and buffalo grass were the dominant feature on the bluffs and tablelands surrounding the valley. Other than the nomadic grazing of bison and other native species, and the cultivation of small patches of corn and squash by Native Americans, there was relatively little disturbance of the prairie. Prairie fires, either intentionally set by Native Americans or naturally ignited by lightning, periodically swept the prairies, returning nutrients to the soil, stimulating the growth of native grasses and forbs and retarding woody vegetation encroachment.
Drainage of river lands began in the 1870s and continues to the present. Most of the large-scale drainage systems were completed by the 1940s. Since then, declining stage levels in the Platte River may have allowed some of the formerly very wet prairie lowlands near the river channel to be converted to croplands. The wet meadows that remain consist of rough topography, or hydric soils that are poorly drained and are not suitable for crop production.
USFWS (1981) and many others have pointed out the crucial importance of wet meadows in the ecology of sandhill cranes and other bird species in the Platte River valley. Currier et al. (1985) chronicled the loss of wet meadow habitats along the Platte and North Platte rivers in Nebraska. Currently, only 27,353 acres of wet meadow vegetation (4.6 percent of all natural and constructed cover types) remain. By river reach, 18,828 acres of wet meadow are in the Big Bend reach and 8,525 acres in the North Platte River reach below Lake McConaughy (Currier et al. 1985).
Because of the importance of wet meadows in the ecology of many bird species, we believe that the most prudent first step is the protection of all remaining wet meadows by state and federal resource agencies and non-governmental organizations. Protection measures that should be used include fee-title acquisition of large (greater than 500 acres) tracts coupled with conservation easements on all smaller tracts. Cooperative management agreements should be obtained among the USFWS, National Audubon Society (NAS), Trust, National Fish and Wildlife Foundation, and TNC. Agreements ensure that all lands controlled by these entities are managed with coordinated habitat goals and objectives. Cooperative agreements could include a description of funding mechanisms for the protection of wet meadows and a schedule for completing habitat protection activities. All acquisitions must be made only on a willing seller basis.
Wet meadows are heavily fragmented. There are few large contiguous tracts of wet meadows. Conservation biologists generally agree that fragmentation of natural habitats is a serious threat to the preservation of biological diversity (Wilcox and Murphy 1985, Noss 1987). Tilman et al. (1994) concluded that even moderate habitat destruction is predicted to cause time-delayed but deterministic extinction of the dominant competitor in remnant patches. The more fragmented habitat has greater numbers of extinctions caused by added destruction. The extinctions are not always noticeable immediately, but occur generations after fragmentation.
Perhaps the greatest challenge in maintaining diversity is the preservation of native biota. We believe that although wet meadows do not support the highest densities of, for example, breeding birds in the Platte River valley, those breeding bird species obligate to wet meadows are probably the least adaptable to habitat change. Species with a narrow niche breadth, such as bobolinks and sedge wrens, cannot readily adapt to habitat conversion.
Noss (1987) outlined several components of the design of natural areas that are applicable to wet meadows. Among the identified components was "bigness" and connectivity. Soulé and Simberloff (1986) concluded that for conservation, large habitat areas are always more valuable than small areas. Furthermore, to facilitate the availability of wet meadow habitats, we recommend that those areas cleared of wooded vegetation (as described above under Mainstem Platte River) be restored to wet meadows. Wet meadow restoration can be accomplished by several techniques. The most important technique is the elimination of wooded vegetation through land clearing and brush removal.
A reliable water supply must be obtained and available for restoration of wet meadows. A proper instream flow must exist to maintain the hydrological ling between river and grounwater levels. Interior (1990) and others have recommended that an instream flow be maintained to carry out the biological processes in adjacent wet meadows. Under Nebraska state law, the NGPC has applied for an instream flow right to protect wet meadows along the Platte River.
Pumping or direct stream diversions during February through April could be used to initiate biological productivity in vegetation. However, the biological effects of such management are not known. The Central Platte NRD and the U.S. Bureau of Reclamation are experimenting with diversion of 1 to 2 cfs from the Platte River near Wood River. Most of the water is used to recharge groundwater with some 200 gallons per minute diverted to a meadow.
Because of the fragmentation of wet meadows, consideration should be given to connecting natural areas by corridors of similar habitat. Noss (1987) noted that scattered islands of similar habitat can be transformed by corridors into larger functional units. Corridors are important to plant and animal species in promoting gene flow and the recolonization of vacant habitats (Wilson and Willis 1975, Brown and Cardiac-Brown 1977, Foeman 1983, Noss 1987).
Specific widths for habitat corridors have not been identified although Noss (1987) stated that "the wider, the better." A wide enough swath creates one large habitat area from two or more smaller areas. Necessary width depends on habitat structure and quality in the corridor, the nature of surrounding habitats, human use patterns and the fauna that are expected to use the corridor (Jahrsdoerfer and Leslie 1988). Because some species for management in wet meadow corridors are obligate plants, insects, and small mammals, we recommend that corridors be a minimum 150 feet wide to ensure the free interchange of plants and pollinating insects. A 150-foot wide corridor should also minimize the effects of ecotonal vegetation that may encroach into wet meadow corridors. We recommend that abandoned roadways, highway rights-of-way, section lines, and railroad rights-of-way within one mile of the Platte River be managed for wet meadow vegetation. We also recommend that resource agencies or non-governmental organizations acquire easements in forests to allow removal of vegetation to enhance wet meadow management.
Sand Pits and Wet Meadow Development
Sand and gravel pits provide some habitat for least terns and piping plovers. The ESA makes no distinction between natural and artificial habitat for conservation of endangered and threatened species. Consequently, sand and gravel companies and public agencies have tried to protect eggs and chicks at sand pit nesting areas from pit operations and recreationists (Cooper and Fries 1993, Sidle and Dinan 1993). Central Nebraska Public Power and Irrigation District (CNPPID), NGPC, Nebraska Public Power District (NPPD), and the Trust, are also coordinating with some sand pit owners to protect nesting areas and to reduce egg and chick predation. These efforts should continue.
There are no local zoning laws applicable to the placement of sand pits. Many sand pits have been constructed in wet meadows adjacent to the Platte River system. Meadows are very important to migratory birds and harbor much of the remaining biodiversity of the central Platte. Most meadows have been converted to agriculture. Remaining meadows are attractive for pit construction because the land is cheaper and more available for purchase than irrigated corn fields, the predominant and highest income-producing land use in the river valley.
Environmental regulations allow federal and state agencies to impart conditions in permits for sand pit construction in wetlands in accordance with Sections 404 and 401 of the Federal Water Pollution Control Act ("Clean Water Act") (33 United States Code 1344) which regulates the discharge of dredged of fill material in the nation's waters. Because few wet meadows remain and their biological value is very high, public resource agencies such as the USFWS usually recommend denial of a permit for proposed activities in wet meadows, including sand pit mining. However, if the U.S. Army Corps of Engineers (Corps) which administers the 404 permit process decides to issue a permit for sand pit construction, resource agencies will recommend that the Corps permit contain special conditions informing the owners that least terns and piping plovers may nest at the site and that nesting birds should be protected from human disturbance. The Nebraska Department of Environmental Quality could also issue conditions in its certification under section 401 of the Clean Water Act (33 United States Code 1341) to protect least terns and piping plovers. Permits are not required for proposed sand pits on grassland or cropland on upland areas.
Section 9 of the ESA prohibits the taking ("harass, harm, pursue, hunt, shoot, wound, kill, trap, capture or collect, or to attempt to engage in any such conduct") of least terns and piping plovers, wherever they occur (Sidle and Dinan 1993). Section 7 of the ESA requires federal agencies to ensure that any action, funded or carried out by them is not likely to jeopardize the continued existence of listed species or modify their critical habitat. For example, if federal funds are being used to purchase sand and gravel from sand pits for road construction and repair, then the federal funding agency may have a responsibility under Section 7 to ensure protection of least terns and piping plovers. It depends upon whether the government is willing to apply the measures of Section 7. Section 7 is best utilized as a conservation tool along with Section 404 of the Clean Water Act by signaling the Corps that should least terns and piping plovers nest at a Corps-permitted sand pit, the Corps, USFWS, and sand pit owner can work together for the conservation of these species.
The best solution to protect least terns and piping plovers at sand pits is to enlist the voluntary cooperation of pit owners. Over the past eight years, owners have allowed agency personnel to survey and study the birds. Moreover, most have cooperated with conservation plans. Conservation activities have included distribution of educational materials, posting of nesting areas, conflict resolution between mining operations and nest sites, predator management, and patrols on weekends to thwart trespassers. We are not aware of any lost income by sand pit owners as a result of measures to protect least terns and piping plovers.
It is not enough to protect least terns and piping plovers by focussing management actions solely on sand pits. To help meet recovery goals of 750 least terns and 280 piping plovers for Platte River, suitable riverine habitat needs to be protected and enhanced (USFWS 1988, 1990). Most least tern and piping plover riverine nesting habitat on the central Platte River has been destroyed as upstream dams and diversions reduced instream flows. Kingsley Dam impounds Lake McConaughy on the North Platte River. Kingsley Dam's hydroelectric project No. 1835 is licensed by FERC and owned and operated by the NPPD. Concern for Least Tern and Piping Plover nesting habitat on the central Platte led FERC to issue terms and conditions to the project's annual license under the authority of the Federal Power Act (16 United States Code 803(j)(1)) which mandates the inclusion of conditions to protect, mitigate, and enhance fish and wildlife affected by the project. Terms and conditions included development and maintenance of eight permanent sites that provide riverine nesting habitat on the central Platte. NPPD has begun construction of sand and gravel islands in the river channel (Plettner 1993) and FERC may order appropriate flows in a new license to NPPD to support riverine nesting and feeding habitat (Lingle 1993a, b, c).
Construction and clearing of islands in rivers for least terns and piping plovers has been carried out in other parts of the birds' breeding ranges (Boyd 1993, Currier and Lingle 1993, Hill 1993, Latka et al. 1993) and is recommended in recovery plans developed under Section 4 of the ESA (USFWS 1988, 1990). On the central Platte, management of terns and plovers at existing sand pits should continue, but management of river flows should also be pursued. NPPD seeks a new license for its hydroelectric project and FERC may require extensive clearing of trees and other vegetation from segments of river channel for a wide variety of wildlife that require open, unvegetated habitats.
* The Opstudy model is a computer program for analyzing the Platte River water resources system. The model was developed by Otradovsky (1986) and refined by CNPPID (1988) and FERC (1992, 1994a, b). The model is used in the FERC re-licensing of Kingsley Dam and associated facilities on the Platte River system. The model includes the North Platte River from Lewellen, Nebraska, the South Platte River from Julesburg, Colorado, and the Platte River from North Platte to Duncan. The output from the model includes monthly flows and summary tables for specific locations for all months and frequency analysis of certain flows and other output.