2.2.2 Geology and Soils

The depth to bedrock within the subwatershed varies from 100-200 feet in the upper subwatershed to 0-100 feet in the lower subwatershed near the Mississippi River.  In the upper subwatershed, quaternary deposits – the surficial material overlaying the bedrock - are generally high relief New Ulm loamy till, with pockets of peat and muck and glacial outwash along the Minnehaha Creek corridor.    In the lower subwatershed, glacial outwash overlays the bedrock with pockets of peat and muck.  Glacial drift deposits up to 300 feet thick lie in the buried bedrock valley under the area near Lakes Calhoun and Harriet.  At Minnehaha Falls and the Mississippi River the glacial drift has eroded away, exposing bedrock.

Soils within the watershed are predominantly urban disturbed soils that have not been classified.  The HHPLS assumed that Urban Disturbed soils were primarily classified as Natural Resources Conservation Service Hydrologic Soil Group B soils. Where the soils have been classified, they are mainly Group B (loamy soils with moderate infiltration potential) and D (clayey soils with very low infiltration potential) (see Figure 3).  The Group D soils are found in low-lying areas and are generally hydric, or showing indications of inundation (see Figure 4) or are in areas of mucky soils.  There are also scattered areas of Group A soils, sandy loam soils with high to moderate infiltration potential.