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Lake Herman, a recreational, shallow, warm water prairie lake near Madison, South Dakota drain in extensively cultivated watershed and acts as a siltation basin for a chain of four lakes. The lake is more than half silted in and since 1968 various rehabilitation efforts have been made, including the construction of silt traps and lake dredging.

Continuation of the monitoring of twenty plus chemical levels and biological populations of green algae, blue-green algae and diatoms has revealed two major changes: (1) ortho phosphate and total phosphorus levels (which increased markedly with the beginning of dredging) have decreased since dredging has ceased, and (2) other parameters including conductivity, chloride, hardness, calcium, magnesium, potassium, and sodium have been gradually increasing since 1968. The former change is probably a direct consequence of dredging and the discontinuation of dredging, but the latter change is more likely to have resulted from natural aging of the lake.

Lake bottom core samples show a pattern of decreasing levels of extractable and total phosphorus with depth. Silt deposit area cores, however, show no consistent variation of phosphorus with depth, and furthermore have significantly lower levels of extractable phosphorus than the upper portion of the lake cores. These observations lend credence to the hypothesis that the increased lake level of phosphorus which occurred with the commencement of dredging activity was the direct result of mechanical mixing of lake water and lake bottom silt.

Measurement of the silt depth at forty sites in Lake Herman and comparison of these measurements with those taken at the same sites in 1967 have revealed that the silt volume of the lake has increased by 2,022 acre feet or 23.8% during the eight years from 1967 to 1975. Experiments done with wheat and barley indicate that the lake silt supports the growth of these grains better than any of the four standard soil types under cultivation on the lake watershed.