Effects of Impoundments on Water Quality of Streams In The Coteau Des Prairies-- Upper Minnesota River Basin

Water-quality and streamflow data were collected in the Coteau des Prairies region of southwestern Minnesota and eastern South Dakota to document the water quality characteristics of streams and impoundments in the Coteau, and to predict the effect of proposed impoundments on the quality of water in Coteau streams. Reconnaissance data collection at 66 stream and 24 impoundment sites plus 21 inlets and outlets during 1979, and intensive data collection at 4 stream and 4 impoundment plus 9 inlet and outlet sites during 1980-84, showed that major ions, nutrients, and suspended-sediment concentrations and suspended sediment discharge differed widely in unimpounded streams, but that maximum and median suspended-sediment concentrations were significantly reduced in impounded streams. Peak daily suspended-sediment discharges were reduced at impoundment outlets relative to the sediment discharge at their inlets. The impoundments were found to have little or no effect on stream temperature and concentrations of dissolved oxygen, dissolved solids, and major ions. Elevated concentrations of fecal bacteria were found in unimpounded streams throughout the study area and the impoundments did not substantially reduce the number of bacteria transported in the impounded streams. During summer, elevated concentrations of nitrate, ammonia, and phosphorus were present in all the impoundments. Levels of productivity were not significantly related to concentrations of total phosphorus in the euphotic zone. Real levels of productivity differed among the impoundments however, and seemed to be affected by the occurrence and duration of thermal stratification. Periods of summer stratification and accumulation of late winter snow on pool ice were frequently accompanied by near total depletion of dissolved oxygen. During summer stratification the concentration of ammonia increased with time in the lower part of the water column in some impoundments.