GEOMORPHIC ASSESSMENT OF KARST TROUT STREAMS IN SOUTHEASTERN MINNESOTA FOR EVALUATING THE EFFECTIVENESS OF AGRICULTURAL BEST MANAGEMENT PRACTICES

The Root River Watershed (RRW) in southeastern Minnesota is a focus of the state’s Clean Water Initiative because of its high-value ecological, recreational, and agricultural resources. Accordingly, several agencies are currently assessing land use impacts on RRW trout streams.

The bedrock in the watershed is predominantly Ordovician sedimentary rocks, including carbonate rocks forming the largest karst terrain in the upper Midwest. The karst terrain poses significant challenges in assessing and managing water resources. In order to provide a baseline for assessing future agricultural BMP implementation, we performed a detailed geomorphic assessment of the morphology and hydraulics of three RRW subwatersheds (Bridge Creek [BC], Crystal Creek [CC], and the South Branch Root River headwaters [SR3]). BC and CC are both heavily karstified watersheds with sinkholes, losing stream reaches, and springs connecting agricultural uplands with trout streams.

Three to five reaches within each subwatershed were surveyed in the field to characterize longitudinal profile, gradient, bedforms, sediment size and distribution, baseflow discharge, and bed shear stress. Within each reach multiple cross-sections perpendicular to flow were also surveyed to delineate bankfull channel dimensions and morphology. RiverMorph^tm software was used to organize and process the stream survey data. During processing, additional morphological characteristics, such as width-to-depth ratio (W:D), entrenchment ratio, and Rosgen stream type were determined.

Reaches in BC are low gradient, meandering, riffle/pool streams with alluvial channels (Rosgen stream types ‘C’ and ‘D’). BC cross-sections display positive relationships between longitudinal distance, W:D, and critical shear stress. Unlike BC, SR3 has a strong relationship between longitudinal distance, the entrenchment ratio, and basal shear stress, but no trend between W:D and critical shear stress. CC shows a negative relationship between longitudinal distance and W:D and a positive correlation between longitudinal distance and entrenchment ratio.

As BMPs are implemented to address “on the farm” erosion and contaminant issues, the baseline stream geomorphic data will provide a yardstick for assessing BMP effectiveness.