SEDIMENTATION IN THE INDIANHEAD RESERVOIR BEHIND ST. CROIX FALLS DAM

Jacque Kutvirt, Department of Geology, Biology, and Environmental Studies, Macalester College, St. Paul MN
Kelly MacGregor, Department of Geology, Biology, and Environmental Studies, Macalester College, St. Paul MN
Dan Hornbach, Department of Geology, Biology, and Environmental Studies, Macalester College, St. Paul MN
Mark Hove, Department of Geology, Biology, and Environmental Studies, Macalester College, St. Paul MN
Byron Karns, National Park Service, St. Croix Falls, WI
Jill Medland, National Park Service, St. Croix Falls, WI

The hydroelectric dam at St. Croix Falls was installed in 1904, impounding the upstream Indianhead Reservoir. Water velocity decreases as the river flows into the reservoir, allowing suspended sediment to settle and become trapped. In addition, bed sediment transport is halted, cutting off source material for the region below the dam. Observations of bed sediment grain size suggest there has been a decrease in grain size of bed sediment at Interstate Park, just downstream of the dam, over the past 20 years (Hornbach and Hove, 2004). In an effort to constrain sedimentation in the reservoir over the past century, and to assess changes in sedimentation rates across the lower portion of the reservoir, we collected sediment cores from three sites in Indianhead Reservoir in winter 2009.

The sediment cores have been examined at the LacCore Lab at the University of Minnesota. They have been (1) logged for density, (2) scanned for magnetic susceptibility, (3) imaged at high-resolution, (4) described on standard Initial Core Description (ICD) sheets, (5) examined microscopically using smear slides, (6) sampled for grain size and mineralogical analysis and (7) analyzed for grain-size of fine sands, silts and clays. The longest cores, collected within 0.5 km of the NPS Headquarters in St. Croix Falls, are 6.5 meters long.

All cores are comprised of fine-grained highly organic mud layers and coarse quartz rich sandy layers. The fine-grained layers likely represent quiescent sedimentation, and the coarse layers represent either annual or stochastic hydrologic or geomorphic events. Future cesium-137 dating of the cores will provide age constraints on sedimentation rates and on the ages of large deposition events. Grain size analysis of the fine-grained sediment suggests that as the reservoir fills, the delta is progressing further downstream over time. The mineralogy, density and magnetic susceptibility of the event-based layers will be used to correlate cores and compare above morphology and mineralogy of reservoir sediment to sediment at Interstate Park.

References:

Hornbach, D.J., and M.C. Hove. 2004. Long-term population dynamics of unionid mussels in the St. Croix River, Minnesota and Wisconsin, USA. Abstracts of the World Congress of Malacology, Perth, Australia, July 11-16, 2004.