Climate as the Main Driver for Rapid Filling of Lake Pepin

Excessive loading of sediment and nutrients to rivers is a major problem in many parts of the United States. One such river in Minnesota is the Minnesota River. Monitoring studies have shown that the Minnesota River contributes approximately 623,000 metric tons per year of total suspended solids to the Mississippi River at Fort Snelling. Sediment core analysis from Lake Pepin, a large floodplain lake (surface area of 103 km2) on the Mississippi River about 80 km south of Fort Snelling shows that the Minnesota River is the major contributor of sediment to Lake Pepin. These core data also show that lake sedimentation rates have been increasing in recent years. Average lake filling rate has varied from 1.15 cm/yr in the mid 1970s to1.6 cm/yr in the mid 1990s. In upper reaches of the Lake, the sedimentation rates were greater than 3 cm/yr over the last three decades. These observations are contrary to trend analysis of Minnesota River water quality which shows that flow adjusted total suspended solid concentrations have been decreasing or stable from 1976-2003. We address the question: why Lake Pepin is filling up at a rapid rate in recent years when flow adjusted TSS concentration have been either decreasing or stable. We show increasing trends in river flows in recent years are contributing to increasing sediment loads in the Minnesota River. We also found a strong correlation between annual river flow and annual precipitation amounts in the Minnesota River basin. Furthermore, we show an increased frequency of larger storms (>5.0 cm precipitation) in the region from the mid-1970s into the mid-1990s. We believe the increased sediment loads in the Minnesota River and indirectly higher sedimentation rates in Lake Pepin are mainly due to differences in the precipitation regime between the mid-1970s and the mid-1990s.