USING GIS TO ASSESS DAM IMPACT ON RIVER BATHYMETRY, ST. CROIX RIVER, MN/WI

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

This study uses Geographic Information Systems (GIS) in combination with field methods and historical maps to assess bathymetric changes to the St. Croix River above the St. Croix Dam over a 40-year period. During the summers of 2006 and 2008, a total of 30,000 sonar depth points were collected between the St. Croix Falls Dam and the Wild River boat launch. The depth data were read into GIS and interpolated to create a raster image of the hypothesized current riverbed. Data from 1968 and 1975 topographic maps were also reformatted for GIS and interpolated to create comparative raster data sets. Map Algebra was used to show how river depth gradients have changed between 1968 and 2008. Water velocity data collected in August 2008 was also mapped in GIS, creating a snapshot of how water velocity along the centerline of the river changes in this region. 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. There is a region of reduced water velocities in the area where infilling appears to be highest. In many places, river shorelines are becoming deeper while the remainder of the channel becomes shallower. In 1968, the volume of the river between the dam and the Wild River boat launch was 10,987 km³. In 2008, the volume was 8,130 km³. This translates into a holding loss of 2,857 km³ of water, or a 26% holding volume decrease. Farther upstream, bathymetric changes appear more typical of an alluvial river. These results may have implications for sediment transport and source below the dam. It has been suggested that changes in sediment composition below the dam has had an impact on the biota of the river, especially on juvenile mussels.