Northern Prairie Wildlife Research Center
U.S. Geological Survey, Water Resources Division, Mailstop 413, Building 53, Lakewood, CO 80225
Geologic deposits in the Cottonwood Lake area are the result of continental glaciation. The glaciers incorporated a considerable amount of shale and silt from bedrock, therefore the drift is very clayey and silty. In addition, intermittent small glacial meltwater streams resulted in scattered sand and gravel deposits within the till. One such deposit underlies the southwest part of the study area and has stringers that extend to the southeast part. Because the till is fine grained, it contains numerous fractures that have variable spacing as close as 100 mm. Many of the fractures are lined with gypsum crystals and many have an iron oxide stain. Iron oxide stain and fine crystalline calcite are common near wetland edges.
Wetlands are confined primarily to depressions in the hummocky landscape of the Cottonwood Lake area. This results in the wetlands having small watersheds and little water contributed from surface runoff. The primary surface runoff is from snowmelt over frozen soils. Therefore, the principal hydrologic components interacting with wetlands in the area are atmospheric water and ground-water. Precipitation is the primary source of water to wetlands in the study area. Most precipitation input is from snowmelt and spring rains, although input from fall rains can also be substantial. Evaporation accounts for the largest loss of water from the wetlands, reaching rates as great as seven mm/day in midsummer for some years.
Wetlands in the study area have a range of functions with respect to their interaction with ground-water. Some seasonal wetlands that are topographically high, such as T8 and T5, recharge ground-water during the brief time in spring when they hold water. Seasonal wetlands that are topographically lower, at an intermediate altitude in the study area, such as wetlands T4 and T3, are flow-through wetlands most of the time. Semipermanent wetlands at an intermediate altitude, such as wetland P1, receive ground-water discharge most of the time. However, the seasonal and semipermanent wetlands at intermediate altitudes commonly have reversals of flow between them and the ground-water system. The topographically lowest wetlands nearly always receive ground-water discharge, but even these have reversals of flow at certain times. The reversals of flow between ground-water and wetlands occur nearly every year and are largely confined to the area adjacent to the wetlands. Ground-water flow toward wetlands is caused by recharge in the upland and by focused recharge near the wetland edge. Flow from wetlands to ground occurs when wetland water levels are higher than the contiguous water table, resulting in bank storage, and by evapotranspiration directly from ground-water at the periphery of wetlands.