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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.

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.

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

Mississippi River Sand Plains Section

Grantsburg Section

Mille Lacs Section

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

Central Plain

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).