WHERE DID ALL THIS SAND COME FROM? USING LEGACY MONITORING DATA AND A GEOMORPHIC SEDIMENT BUDGET TO DETERMINE SAND LOADING IN THE GREATER BLUE EARTH RIVER BASIN

Sediment budgets provide an accounting of sources and sinks for sediment in a watershed, providing actionable information to watershed managers tasked with improving water quality and watershed health. Here we report on a sand budget developed for the Greater Blue Earth River basin in south-central Minnesota, USA, using two methods. The first method built upon a pre-existing fine sediment budget to predict sand loading. This fine sediment budget quantified four sediment sources (bluffs, ravines, streambanks, and primarily-agricultural uplands) and two sediment sinks (floodplains and upland lakes), by determining the mass of silt and clay entering or leaving the main channel in an average year. The sand budget uses field-determined grain size distribution data from major sediment sources to compute inputs of sand rather than fine sediment and adjusts upland lake trapping efficiencies to fully capture any sand entering lakes. The second method uses extensive monitoring data for total suspended solids (TSS) collected by the Minnesota Pollution Control Agency and partner agencies. Using existing studies where TSS and suspended-sediment concentrations (SSC) data were both collected, we developed relationships to predict sand concentrations from TSS concentrations. Annual sand loads were calculated from 2007-2019 and compared with predictions from the geomorphic sediment budget. Over this time period, average sand load predictions from the Blue Earth, Watonwan, and Le Sueur Rivers combined were 422,000 Mg/yr derived from TSS monitoring, and 290,000 Mg/yr from the geomorphic sand budget. This is compared to an average TSS load over this same period of 483,000 Mg/yr, indicating that the sand load is similar in magnitude to the fine sediment load. This approach shows the potential to use previously collected TSS data to determine sand loading.