4 



NORTH AMERICAN FAUNA 73 



the combined action of climate, vegetation, and proper acidity of these gla- 

 cial deposits, soils developed that are very important for agricultural pro- 

 duction. Productivity of all soils in the Valley is, of course, not equal. Soil 

 conditions range from the sandy, acidic beach-derived soils associated with 

 Glacial Lake Grantsburg, to the deep, rich loams and silt loam prairie soils 

 of the lower Valley. Productivity of these soils for wildUfe appears to corre- 

 late nicely. For example, the acidic soils of Glacial Lake Grantsburg are 

 usually associated with relatively sterile Jack Pine Barren forest which sup- 

 ports limited wildUfe populations. On the other extreme, the deep, rich 

 prairie soils of the southern regions produce exceptional agricultural crops 

 and also support large and highly diverse wildUfe populations. 



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 miUion 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 pregla- 

 cial 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 over- 

 flow 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 VaUey is composed primarily of broad, flat 

 glacial outwash plains and kettle and knob topography associated with ter- 

 minal 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 alkaUnity and a deep brownish stained color 

 (Waters 1977). In the southern reaches of the Valley, alkaHnity and vegeta- 

 tion of lakes and marshes become progressively greater owing primarily to 

 limestone and sandstone bedrock. 



The entire St. Croix watershed encompasses about 11,550 km^ (1.7 mil- 

 Uon ha). The river drains about 7,233 km^ in Wisconsin and 4,317 km^ in 

 Minnesota (Lindholm et al. 1974). Included in the watershed are aU or part of 



