TRACKING SEDIMENT FROM SOURCE TO SINK: SEDIMENT FINGERPRINTING AS A TOOL TO GUIDE WATERSHED MANAGEMENT IN THE LITTLE FORK RIVER

Fine sediment is a leading cause of habitat degradation in streams across the globe. Over the past several decades, the need for detailed information describing sediment sources and transport pathways has spurred the development of methods for tracking sediment from source to sink. Sediment fingerprinting, which uses geochemical tracers to tie fluvial sediment to its source in the landscape, has become an EPA-certified tool for sediment source delineation, and has been applied across a wide range of settings and at varied spatial scales. In this presentation, we will describe sediment fingerprinting methods broadly and present plans for their application in the Little Fork Basin, Minnesota.

The Little Fork River is a disproportionate contributor of sediment to downstream Rainy River and Lake of the Woods, both of which are internationally managed waters suffering from excess sedimentation and eutrophication respectively. The Minnesota Pollution Control Agency has identified turbidity and fish community impairments on several reaches of the Little Fork River, and the development of Total Maximum Daily Load regulations is presently underway. Determination of fine sediment sources is a critical early step to developing strategy to meet their target of 45 mg/L of total suspended solids.

To delineate the dominant sources of fine sediment to the Little Fork Basin, a study was initiated in fall of 2019 which will utilize a combination of geochemical sediment fingerprinting and sediment budget techniques. Sediment fingerprinting will use a set of elemental tracers to determine the proportion of fine sediment from distinct erosional source areas in uplands and along the stream corridor that contribute to fluvial suspended and bed sediment. Upland sources investigated will include mature forests, recently harvested forest, wetlands, roadways and cropland. Stream corridor sources explored will include streambanks, eroding valley walls, and gullies. As a complimentary line of evidence, a sediment budget based on field and geographic information system analysis will also be constructed for stream corridor sources of sediment. The results of this study, which are anticipated by fall of 2021, will be used to help target mitigation strategies to reduce erosion from the heaviest contributors in the Little Fork River Basin.