SAND BUDGET FOR THE LITTLE FORK RIVER, A LOW-GRADIENT POST-GLACIAL RIVER IN NORTHERN MINNESOTA, USA

Geomorphic sediment budgets are a common way to quantify sediment loading of a river and serve to focus management actions and restoration funding where it can be most effective. Budgets like this rarely incorporate coarse grain sediments (sand) due to complexities related to sediment delivery and longer transport times. These complexities, however, may be used to describe a longer timescale of geomorphic impact. The Little Fork River in northern Minnesota, USA, was intensively logged in the late 1800s and early 1900s with log drives lasting through the 1940s. It has since been identified as a significant contributor of fine sediment and potentially phosphorous to the Rainy-Lake of the Woods Basin downstream. The Minnesota Pollution Control Agency recently set a turbidity target with a need for additional information on where sediment is being sourced in the watershed. This project seeks to augment the fine sediment budget being conducted by U.S. Geological Survey (USGS) by adding in sand, defining its differing origins and estimating its transport times. In addition to USGS sampling efforts of major sources, targeted sampling was completed for ravines, floodplains and culvert road crossings, to quantify sand storage across the watershed. Erosion rates were determined as part of the USGS study through field-based rapid geomorphic assessments. Remote analyses of lidar topography, land cover, and surficial and bedrock geology were used to extrapolate these erosion rates across the basin to determine watershed-wide sediment contributions. These sediment data were then combined with grain size analyses to determine sand-specific contributions. The sand budget was then used to feed a sand transport model with stream reach characteristics. Initial findings indicate that erosion in the riparian corridor is most tightly correlated to high slopes near the channel, with main-stem bank erosion coupled to valley geometry. The sand is predominantly sourced from the stream corridor, with over a third originating from first order ravines. Transport estimates show that low gradient channel segments act to limit overall transport capacity suggesting that the system is ultimately transport limited.