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Riparian Ecosystem Creation and Restoration:
A Literature Summary

Introduction

Status of Riparian Ecosystems in the United States


Over half of the wetland and riparian zones have been destroyed in the coterminous 48 States, and few remaining zones have not been adversely impacted (Fredrickson and Reid 1986). Because of their location in floodplains, destruction of riparian ecosystems is largely associated with man's activities, especially clearing for agriculture, stream-channel modifications, water impoundments, and urbanization. Narrower riparian areas are more easily altered and potentially degraded. Because riparian zones often follow the gradual elevational changes of a watershed, road and pipeline construction often impacts riparian ecosystems. Even recreational development can destroy natural plant diversity and structure, lead to soil compaction and erosion, and disturb wildlife.

Native riparian ecosystems, especially in the arid Southwest, are disappearing rapidly (Ohmart et al. 1977). About 1,200 ha/year of riparian vegetation are being removed along the lower Colorado River (Anderson et al. 1979). Many cottonwood (Populus spp.) communities of this area have been lost to expanding agriculture, livestock grazing, channelization projects, stream dewatering, inundation by reservoir construction, and groundwater depletion. Dams also have expedited the natural loss of riparian communities by stopping annual flooding. Cessation of annual floods and natural channel movements curtail the formation of the basic cottonwood seedling habitat--bare sandy soils with high water tables--which appears to be essential for seed germination. In addition, domestic livestock concentrate in riparian communities and heavily graze young cottonwoods.

Riparian areas are widely recognized as crucial to the overall ecological health of rangelands in the western U.S.; however, many are in degraded condition, largely as a result of poorly managed livestock grazing (U.S. General Accounting Office 1988). Riparian areas represent only about 1% of the more than 250 million acres of Federally owned rangeland. Riparian areas, however, have ecological importance far beyond their relatively small acreage because they have a greater quantity and diversity of plant species than adjoining land. Livestock tend to congregate in riparian areas for extended periods, eat much of the vegetation, and trample streambanks, often eliminating other benefits of riparian habitat (e.g., fish and wildlife habitat, erosion control, floodwater dissipation).

In the Pacific Northwest, stream corridors are major sources of erosion (Carlson 1979). Human activities such as logging, urban development, grazing, cropping, and recreational activity have increased surface runoff, removed protective riparian vegetation, and altered flows, often with catastrophic effects. About 10 million tons of sediment erode from streambanks each year in Oregon and Washington alone.

Mining activity, especially in the East, has destroyed many riparian habitats. For example, central Florida's phosphate district has been surface mined since 1908, and by 1975 over 60,000 acres (much of which was wetland) had been abandoned without reclamation (Clewell 1983). When reclamation of active mines became mandatory in 1975, most mine cuts were filled with overburden, sand tailings, and waste clays. Insufficient material remained to fill all mine cuts to the original grade of the land; thus, some cuts remained as relatively deep lakes, whereas others were filled and reclaimed as uplands. Reclamation of riparian and wetland habitats in Florida's phosphate district has been attempted only since 1978.

In the Southeast, over 90% of original bottomland forest has been converted to other land uses, primarily agriculture (Haynes and Moore 1988). Some major river systems also have had a long history of land alterations associated with flood control measures. Since the late 17th century, levees and borrow pits have been constructed along the lower Mississippi River (Landin 1985). The 600 miles of the river between the Gulf of Mexico and Cape Girardeau, Missouri, is contained by over 2,000 miles of levees and has about 40,000 acres of borrow pits from which the levee material was taken. As native riparian landscapes have been increasingly impacted by flood control projects, the need has grown for restorative mitigation, not only along existing rivers and streams but also on newly created floodways and distribution canals (Dawson 1984).

In urban communities one of the major problems facing local governments involves an economically and environmentally acceptable solution to increased flooding of urban streams (Keller and Hoffman 1976). Flood correction and control typically includes stream channelization (i.e., widening, straightening, or deepening a stream channel). Channelization tends to adversely affect the physical and biological environment and to reduce the aesthetic quality of the stream.


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