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Birds of the St. Croix River Valley: Minnesota and Wisconsin

Physiography and Land Use

The diverse physiographic and topographic features of the Valley result from at least four different glacial epochs that extended from 1 million to 10,000 years ago. The St. Croix River itself is an important component of past and present physiographic changes. The gradient of the river averages 102 cm per km across its entire course, ranging from nearly zero on Lake St.Croix to 4.3 m per km at the Kettle River Rapids, Pine County (Young and Hindall 1973). Altitude of the land surface ranges from about 207 m above sea level at Prescott to about 518 m near Cable at the upper end of the Namekagon River. The interior of the Valley has a general slope ranging from 427 m in the north to about 305 m in the south.

During the last glacial period the St. Croix served as a major drainage for glacial melt waters. Martin (1932) reported that the post-glacial course of the river contained the Apple River Valley in western Wisconsin. In preglacial times, the major course of the river was farther west in Minnesota. In addition to carrying melt waters from the glacier, the St. Croix carried overflow waters from large glacial lakes that occurred in the Lake Superior basin. One outlet was the valley at Solon Springs which is now occupied by St. Croix Lake, the headwaters of the river (Young and Hindall 1973).

Topography of the Valley includes flat-topped, steep-sided sedimentary hills adjacent to the river, and narrow stream valleys in the extreme lower reaches. The remainder of the Valley is composed primarily of broad, flat glacial outwash plains and kettle and knob topography associated with terminal and end moraines.

Lakes and marshes are abundant in the northern half of the Valley where surface drainage systems are poorly developed. One unique drainage system in southern Pine County is separated from all other watersheds in the region by a zone of elevated igneous bedrock. Marshes and streams in this region are characterized by low alkalinity and a deep brownish stained color (Waters 1977). In the southern reaches of the Valley, alkalinity and vegetation of lakes and marshes become progressively greater owing primarily to limestone and sandstone bedrock.

The entire St. Croix watershed encompasses about 11,550 km2 (1.7 million ha). The river drains about 7,233 km2 in Wisconsin and 4,317 km2 in Minnesota (Lindholm et al. 1974). Included in the watershed are all or part of 18 counties, 9 in each State (Fig. 1). There are eight major streams or watersheds that are tributary to the St. Croix, including the Snake and Kettle rivers, and Pine County streams in Minnesota (Waters 1977), and the Namekagon, Clam, Apple, Willow, and Kinnickinnic rivers in Wisconsin.

GIF-Map of St. Croix Watershed
Fig. 1. The St. Croix River watershed, including Minnesota and Wisconsin, encompasses nearly 11,550 km2.

Related to the tremendous amount of diversity and variability in the topography and geology of the St. Croix River Valley, various geographers and plant ecologists have developed systems for classifying landforms and major vegetative communities (Fig. 2). These systems have provided divisions that are important in understanding the interrelation of bird distribution with vegetation and geomorphic features. Two separate systems have been developed, one for each State. Both systems have considerable merit considering the foundation upon which they were based. Unfortunately, these separate systems make interpretation of bird distribution awkward when considering the two States. Following is a brief overview of these major divisions by State and the reasons for the combining of systems that I had to consider for interpretation.

GIF-Map of physiographic regions of St. Croix valley
Fig. 2. Major physiographic regions of the St. Croix River Valley.

The most usable division system for Minnesota was developed by Kratz and Jensen (1977). They developed a system based on both major vegetational and geomorphic provinces and included 17 distinct Ecological-Geomorphic divisions. Four of these divisions are included in the Minnesota counties. A brief description of each follows.

Southern Oak Barrens Section

This section includes a large region of southeastern Minnesota generally referred to as the Driftless Area, which was not covered by ice during glacial periods. In the Valley, about 80% of Washington County is included in this section. Topography is characterized by highly dissected limestone and sandstone hills adjacent to the St. Croix River. Away from the river, large expanses of glacial outwash and kettle and knob topography predominate. Vegetation consists of a transition between prairie and deciduous forests. Originally, oak barrens and Savannah occurred throughout this area.

Mississippi River Sand Plains Section

This section includes a small portion of northwestern Washington and southwestern Chisago counties where topography is characteristic of glacial outwash plains - including broad, flat areas with occasional wetland basins interspersed. Original vegetation of the sand outwash soils was primarily oak forest with scattered prairie openings and wet prairie in depressions.

Grantsburg Section

This section includes most of Chisago County and a small part of southern Pine County. Topography is characterized by flat to gently rolling glacial sediments that are frequently dissected by streams. The Grantsburg section includes the bed of Glacial Lake Grantsburg, which extended eastward into Burnett County, Wisconsin. Vegetation originally included Jack Pine Barrens and scattered oak openings. Currently, much of the land is in agricultural production.

Mille Lacs Section

This section includes most of southern and all of northern Pine County. Topography is characterized by glacial scoured bedrock and glacial till that provide a kettle and knob topography. Forest-bordered lakes are numerous throughout this section. Vegetation is characterized by mixed northern hardwood and coniferous stands with conifer bogs interspersed throughout.

The classification system used in Wisconsin (Martin 1932) is based primarily on physiographic features and bedrock geology, regardless of vegetation. However, the influence of physiography on vegetation is well demonstrated in Wisconsin because the regions that occur in the Valley each support entirely different vegetative communities. Martin considered five distinct physiographic provinces in Wisconsin and the Valley is included in three.

Western Upland

This region in the Valley is the northern extension of the Driftless Area, a part of Wisconsin, Minnesota, and Illinois that was not covered by glacial ice. Included in the Western Upland are the highly dissected stream valleys adjacent to the St. Croix River and the broad and gently rolling glacial outwash plain of central St. Croix County. Original vegetation included Southern Oak Forest in the dissected area and Tall Grass Prairie associated with the outwash plain. At present, much of the Western Upland is in agricultural production. Portions of central St. Croix, southern Polk, and central Washington counties are dotted with numerous natural basin wetlands.

Central Plain

This region includes northern St. Croix, southern Burnett, and most of Polk counties. Topography of the Central Plain is also characterized by broad glacial outwash plains lying over Cambrian sandstone in the southern regions. Extensive areas of kettle and knob topography associated with end moraines occur in the north. Numerous lakes and bogs are associated with this topography. Vegetation consists of a mixture of southern oak forest, prairie and northern mixed hardwood, and coniferous forest.

Northern Highland

This region, which includes northern Burnett and southern Douglas counties, has topography characteristic of morainal deposits and includes numerous high, rounded hills composed of more resistant bedrock. Geology of this region is primarily highly resistant Precambrian igneous rock, quite unlike southern regions of the Valley. Original vegetation of the Northern Highland was composed almost exclusively of pine forest and coniferous bogs. Presently, the pine forest exists only as scattered remnants interspersed throughout extensive mixed second growth deciduous and coniferous forest. Large lakes and extensive bogs are characteristic of the Northern Highland.

This brief discussion of the major physiographic regions of the St. Croix River Valley points out both similarities and differences between the systems developed for each State. For the sake of convenience and clarity in describing bird occurrence and distribution, I have combined both systems. Thus, throughout this report reference to the Western Upland will include the Wisconsin areas outlined in Martin (1932) and the Southern Oak Barrens region of Minnesota described by Kratz and Jensen (1977). The Central Plain will include the Wisconsin areas outlined and those areas of Chisago and Washington counties included in the Mississippi River Sand Plains and the Grantsburg Section. Reference to the Northern Highland will include the Wisconsin areas and the Mille Lacs Section in Pine County. These agree closely with the physiographic regions used by the U.S. Fish and Wildlife Service's Breeding Bird Survey (BBS; Bystrak 1979).

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