SPATIAL AND TEMPORAL VARIABILITY IN SUSPENDED SEDIMENT IN THE ST. CROIX RIVER, MN/WI

Robin Major, Kelly R. MacGregor, Mark C. Hove and Daniel J. Hornbach
Departments of Geology, Environmental Studies and Biology, Macalester College

Suspended sediment in rivers is critical to controlling nutrient and contaminant transport, penetration of light into the water column, and fluvial ecosystem health. Quantifying suspended sediment over monthly to yearly timescales can be challenging. Though suspended sediment loads fluctuate by season, a reliable sediment load is essential for maintaining a stable ecosystem. Past research in the St. Croix suggests that changing patterns of sediment deposition may impact the diverse mussel community in the river. The goal of this project is to assess whether water discharge controls suspended sediment concentration in the St. Croix River. In particular, we are interested in variability in sediment concentrations above and below the St. Croix Falls Dam. For 6 months water samples were collected from four sites: two above and two below the St. Croix Falls Dam. Rating curves, showing the relationship between water discharge and suspended sediment concentration, show that suspended sediment concentration was sensitive to water discharge. A strong positive correlation exists above 2500 cfs between suspended sediment and discharge. Sediment concentrations varied with water discharge at all four sites, with the lowest concentrations at the National Park Headquarters within the reservoir above the dam, and the highest concentrations below the dam at the USGS gaging station. Hysteresis occurred at all four sites, with suspended sediment concentrations greatest on the rising limb of the annual hydrograph. Estimates of the annual suspended sediment loads at each of the four sites suggest sediment is deposited into the reservoir. Further sampling and analysis will focus on the source of increased sediment in suspension at Interstate Park (below the dam).