Historical sediment flux from three watersheds into Lake Pepin, Minnesota, USA

This contribution presents an assessment of the modern and historic fluxes of sediments exiting the Mississippi, St. Croix, and Minnesota watersheds and entering Lake Pepin, a natural riverine lake on the Upper Mississippi River. A Chemical Mass Balance receptor model and elemental signatures for sediments in Lake Pepin and the rivers draining its three principal catchment areas were used to apportion sediments in the lake to their source watersheds. The apportionment data, estimated trapping efficiency of the lake, and watershed basin areas were used to calculate sediment fluxes and erosion rates for each basin on a decadal basis. The relative apportionment of sediments from the Minnesota River watershed increased since European settlement of the region circa 1830 from 83 to 87% for the upper, 83 to 90% for the middle, and 78 to 87% for the lower reaches of the lake. Sediment loading to the whole lake shows a 12-fold increase from historic levels in the mass of Minnesota River-derived sediments. The amount of sediment currently supplied by this river is more than seven times the amount supplied by the headwater-Mississippi and St. Croix Rivers combined. Sediment contributions from these two combined watersheds have risen sevenfold since settlement. The causes of these increases are attributable to intensive agricultural production, especially within the Minnesota River basin. Watershed alterations have resulted in a decrease in wetlands, riparian zones, and native prairie and an increase in acreage artificially drained by sub-surface tile lines and surface ditching.