BIRDS OF THE ST. CROIX RIVER VALLEY 



3 



Wetland habitats have also been significantly reduced. Peterson (1978) 

 showed that over 27% of the seasonal wetlands (Type III of Shaw and 

 Fredine 1956) in St. Croix County were drained between 1958 and 1977. 



The objective of this report is to provide, in condensed form, an account of 

 the 314 bird species that have been recorded in the Valley. 



Climate 



The Valley has a temperate, continental climate that is characterized by 

 extreme seasonal variability. Mean monthly air temperatures are -12 to 

 -9°C in January and 20 to 22 °C in July. The average growing season ranges 

 from 100 days in the north (Burnett and Pine counties) to over 120 days in 

 the south (St. Croix and Washington counties). The ground is frozen usually 

 from late November through mid- April and frost depths range from 66 to 

 86 cm. 



Mean annual precipitation is 74.4 cm (Young and Hindall 1973). The range 

 in yearly precipitation is about 71.1 cm at St. Croix Beach, Washington 

 County, to nearly 78.7 cm at Grantsburg, Burnett County (Lindholm et al. 

 1974). Periods of prolonged drought are highly infrequent. February is 

 usually the driest month (< 2.5 cm) and June the wettest (12.2 cm). Annual 

 evapotranspiration from the St. Croix watershed is 59.4 cm (Lindholm et al. 

 1974). During July and August, when temperatures are highest and plant 

 moisture demands are greatest, evapotranspiration often exceeds precipita- 

 tion. 



Geology 



Interpretations of the interrelation of birds and other wildlife with their 

 environment require an understanding of the geology and soils of an area. 

 The St. Croix River Valley is an area rich in geologic history. Sediments and 

 rock formations throughout the Valley range in geologic age from the 1-bil- 

 hon-year-old Precambrian lava flows at Interstate Park to very recent sedi- 

 ment deposition at the mouth of the Kinnickinnic River. 



Bedrock in the upper portion of the Valley consists primarily of Pre- 

 cambrian igneous lava flows, shales, sandstones, and igneous and meta- 

 morphic crystalline formations. Bedrock of the lower portions of the Valley 

 consists of marine sandstones, shales, and Umestones that were deposited 

 during the Cambrian and Ordovician periods. Outcroppings southward 

 through the Valley indicate that bedrock formations are progressively 

 younger. 



Glacial deposits consist mainly of undifferentiated till that was deposited 

 as the last glacier melted about 10,000 years ago. This glacial action was 

 very important in forming the current features of the landscape. Especially 

 important and characteristic among these glacial deposits are the extensive 

 sandy soils associated with the bed of Glacial Lake Grantsburg in Burnett 

 and Pine counties, and the numerous prairie pothole- type wetlands that 

 occur in Polk, St. Croix, and Washington counties. 



One important result of past glacial activity in the St. Croix River Valley 

 is the highly productive soils that formed on the glacial outwash. Through 



