Sediment Transport in the St. Croix River, MN/WI Above and Below the St. Croix Falls Dam
Abstract
Sediment budgets in river networks are notoriously difficult to construct but key for quantifying both short and long-term changes to fluvial environments. Hydrologic and sedimentologic conditions in the St. Croix River, one of the first rivers in the US to be designated a National Wild and Scenic River, play a significant role in the stability of native freshwater mussel populations. Data show a decrease in the grain size of bed sediment and a >90% decline in the juvenile mussel population in the last decade, but only in a region below the St. Croix Falls dam, the major dam in the river system. The area is home to two federally endangered mussel species; we need to better understand the controls on sediment transport into and out of this stretch of river to understand the causes for the mussel decline. We are working toward constructing a sediment budget (both suspended and bedload) in the region bracketing the St. Croix Falls dam. In conjunction with mussel habitat analyses, we collected surface and near-bed suspended sediment, as well as bedload transport samples since summer 2004. Beginning in January 2008, we collected suspended sediment samples weekly at four locations, two above the dam and two below. Comparison to data collected in the 1970's suggests a decrease in suspended sediment over time. We measured near-bed sediment transport at a sandy habitat upstream of the St. Croix Falls dam, and a rockier habitat downstream of the dam. A significant relationship was found between water discharge and bed sediment transport at both high (>5000 cfs) and low flows at both sites. Using historical maps and recent bathymetric data, we used GIS to construct changes in reservoir volume over time. Our results indicate that there has been substantial sediment infilling behind the dam over the last 40 years. The river has deposited as much as 8 meters of sediment in some places since 1968. This translates into a 26% holding volume decrease. These results may have implications for sediment transport and source below the dam.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2008
- Bibcode:
- 2008AGUFM.H53C1069M
- Keywords:
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- 1808 Dams;
- 1813 Eco-hydrology;
- 1815 Erosion;
- 1825 Geomorphology: fluvial (1625);
- 1856 River channels (0483;
- 0744)