SUSPENDED SOLIDS TRANSPORT ON THE ST. CROIX: IMPACTS OF THE ST. CROIX FALLS DAM

C. Skarbek, Macalester College
K. MacGregor, Macalester College
D. Hornbach, Macalester College
M. Hove, Macalester College

Transport of inorganic and organic particles in the water column is a key process for the 40 species of filter feeding freshwater mussels that inhabit the St. Croix River, nearly half of which are endangered or threatened. Structures like the St. Croix Falls Dam can impede suspended solid transport, affecting downstream habitat. Due to a decline in juvenile mussel density below the St. Croix Falls Dam over the past two decades (Hornbach and others, 2009) we are interested in spatial variability in suspended matter as well as how suspended solid concentrations change during the annual hydrologic cycle. This project examines the quantity of suspended sediment and organic matter in the river at four sites, two above and two below the St. Croix Falls hydroelectric dam, during the annual flow cycle from 2008-2010. We are interested in locating sources and sinks of suspended material between Nevers Dam and Franconia, and seek to understand factors controlling the entrainment and transport of suspended material. We considered rainfall events and water discharge as possible drivers of variability in suspended solids concentration. Analysis of suspended solids samples from the past two years demonstrate there is not one single factor controlling the transport of suspended solids. Between January 2008 and June 2009, suspended sediment concentration (SSC) closely tracked water discharge at all four sites. However, between Summer 2009 and Summer 2010 water discharge was not a good predictor of SSC, with peak concentrations occurring at relatively low discharges. Overall we found water discharge to be the best predictor of SSC, though the concentrations in 2009 and 2010 suggest that there are also other factors at work. Rainfall records from Wild River State Park were examined to see if the amount of rain, which is associated with sediment runoff and SSC flux, was a good predictor of SSC spikes. There were some loose associations, but no strong correlation. There were no statistically significant differences found in surface SSC at the four sites examined, suggesting that the dam practices being implemented have limited impact on the transport of the finest suspended materials. The average amount of total suspended solids (TSS) through the two year period of this study was 0.0086 g/L, nearly 36% of which was found to be organic. The TSS increased from 2008 to 2009 by 0.0005 g/L, a significant amount, considering the size of the St. Croix. Looking more closely at weather records and other possible variables including development practices and storm events will allow us to better constrain the controls on suspended solids transport over the past two years.

Suggestions for Reading

Hornbach, D.J., Hove, M.C., MacGregor, K.R. (2009) Mussel density at Interstate Park, St. Croix River, MN and WI: A new equilibrium? International Symposium of the Freshwater Mollusk Conservation Society, Baltimore, MD, April 19-24, 2009.