Using Fine Sediment Monitoring to Quantify Sand Loading Over Time

Riverine sand loading can lead to geomorphic change, impacting river processes like lateral migration and vertical aggradation or degradation. Yet many sediment monitoring programs track only fine sediment measurable as total suspended solids (TSS). Here, we assess the ability for TSS data to be used to determine sand loading. We take advantage of published sampling by the US Geological Survey (USGS) of suspended-sediment concentration (SSC) combined with TSS data to develop relationships between TSS and sand loads for the Greater Blue Earth River (GBER) basin in south-central Minnesota, USA. The GBER basin has some of the highest reported TSS loads in the state of Minnesota, and significant research has gone into delineating a geomorphic fine sediment budget for the watershed. We used this fine sediment budget to construct a sand budget using field-determined grain size distribution data from major sediment sources and adjusting upland lake trapping efficiencies. The geomorphic sand budget predicts 290,000 Mg of sand transported out of the GBER basin each year. In comparison, annual sand loads derived from 2007 to 2019 TSS data were 359,000 Mg/year, an average % difference of only 22%. The average annual TSS load during this same time period was 469,000 Mg. If only TSS monitoring is used to assess sediment loading from this watershed, it could underpredict the suspended sediment fraction by 36%–42%. This study highlights the potential to use TSS monitoring data to track sand loading over time as well as the importance of including sand as part of suspended sediment sampling.