Using GIS to assess dam impact on river bathymetry, St. Croix River, MN/WI

This study uses GIS in combination with field measurements and historical maps to assess bathymetric changes in the St. Croix River flowage above the St. Croix Falls Dam over the last 40years. During the summers of 2006 and 2008, a total of 30,000 sonar depth points were collected in the flowage, read into GIS and interpolated to create a raster image of the current riverbed. Data from 1968 and 1975 topographic maps were digitized and interpolated to create comparative raster data sets. Map Algebra was used to calculate changes river depth gradients.

Our analysis indicates there has been substantial sediment infilling behind the dam during the last 40 years with as much as 8 meters of sediment deposited in some places. In 1968, the volume of the river between the dam and the Wild River boat launch was 10,987 km³ and decreased to 8,130 km³ in 2008. This translates to a 26% holding volume decrease. These results may have implications for sediment transport and source below the dam. Changes in sediment composition below the dam have occurred concurrently with significant changes in juvenile mussel density.