Climate and Channel Modifications as Probable Causes for Increased Sedimentation in Lake Pepin.

Sediment pollution is one of the major causes of water quality impairment in many surface water bodies. Lake Pepin, a large floodplain lake on the Upper Mississippi River bordering Minnesota and Wisconsin is one such water body. Most of the sediments in this lake come from the Minnesota River Basin and more specifically from a relatively flat Greater Blue Earth River Basin (GBERB). Since 1900s, sedimentation rates in Lake Pepin have increased which has been attributed to the presence of surface inlets and tile drainage in the basin. Since all rivers in the GBERB are deeply incised and have unstable banks, this study quantified bank erosion from various rivers in Blue Earth County using LiDAR (Light Detection And Ranging). Fine sediment (silt +clay) contributions from river banks vary between 69% and 86% of the transported sediments. Precipitation appears to be a major factor in controlling sediment production. Explorer’s accounts in 1835 indicate that GBER was muddy and was the cause of substantial turbidity of the Minnesota River. In 1893, the US Army Corps of Engineers constructed a dam at the mouth to raise the water level for excursion boats. Between 1910 and 1940, precipitation also steadily decreased. Historical records also show that mouth of the Minnesota River was partially blocked each spring by sand bars before 1940. All this suggests that much of the earlier sediments did not get transported to Lake Pepin and are deposited somewhere between Mankato and Minneapolis. In 1909, the dam was removed and starting in 1892 the Lower Minnesota River (LMR) was deepened to 4, 5, and 9 ft depths. Starting 1940s, precipitation also steadily increased.  This suggests that increased sedimentation in later part of last century in Lake Pepin is probably due to combined effects of deepening of LMR and wet climate since the dust bowl days.