Northern Prairie Wildlife Research Center
U.S. Geological Survey, Water Resources Division, Federal Building, Room 321, P.O. Box 1437, Grand Forks, ND 58201; University of North Dakota, Department of Geology and Geological Engineering, University Station, Box 8358, Grand Forks, ND 58202
Large freshwater wetlands occupied many parts of the Red River of the North valley prior to 1940. Most of these wetlands have been drained and placed into agricultural production. However, many smaller wetlands still occur west of the Red River of the North. In contrast to most wetlands in the Upper Midwest, soils and water in these small wetlands have large sodium and chloride concentrations that limit their use for agricultural production. The purpose of this paper is to describe hydrogeologic conditions that are responsible for forming these saline wetlands in the Red River of the North valley.
The locations of saline wetlands generally correspond to the subcrop area of the Lower Cretaceous and Paleozoic bedrock beneath variably thick glacial and lacustrine sediments. Ground-water discharge from the bedrock aquifers occurs as part of a regional flow system that extends westward from the Red River of the North valley to the Rocky Mountains. Water in the wetlands originates from precipitation and from saline ground-water discharged from deep bedrock aquifers and shallow glacial aquifers.
Three hydrogeologic processes, which are related to texture of sediments, topographic depressions, and flowing wells, could be responsible for the formation of the saline wetlands. In areas where the most extensive saline wetlands occur (near Kelly's Slough, Lake Ardoch, and Salt Lake), the wetlands presumably overlie coarse-textured glacial and lacustrine sediments that allow for significant areas of increased vertical flow. In particular, the topography of Kelly's Slough may indicate significant erosion of fine-grained lacustrine sediments by ground-water discharge early in the history of the wetland.
In other areas, wetlands could have been developed in naturally occurring depressions by direct precipitation, surface inflow, and discharge of ground-water from shallow aquifers. This generally occurs where the fine texture of glacial and lacustrine sediments limits the volume of discharge from the bedrock aquifers to the surface. The only discharge from these topographic depressions is to evapotranspiration. Evapotranspiration is greater than precipitation and results in a seasonal deficit. This deficit, along with the slow discharge of deep, saline ground-water to the depressions, has led to large concentrations of dissolved solids. Seasonal fluctuation of dissolved solids in water in the wetlands may be as much as 20,000 mg l-1.
Several small saline wetlands may have resulted from flowing wells. Numerous flowing wells were completed in bedrock aquifers during the first half of this century in eastern Grand Forks and Walsh counties. The distribution of these wetlands is restricted to areas where the potentiometric surface of the bedrock aquifers is above the land surface.