Watonwan River Watershed Hydrology, Connectivity, and Geomorphology Assessment Report

The Watonwan River watershed is an eight-digit Hydrologic Unit Code (HUC) watershed draining approximately 878 mi2 of predominantly agricultural land in south-central Minnesota. The following report analyzes the hydrology, connectivity, and geomorphology components of the Watonwan River watershed. Historical gage data on the Watonwan River, stream crossing data, and applied fluvial geomorphology assessments were analyzed to characterize conditions of the watershed and find relationships to help understand water quality and biological impairments throughout the watershed. Discharge and precipitation data collected from the Watonwan River indicate the amount and timing of water delivered per inch of precipitation has increased over time. This increased volume and rate of water delivery can further destabilize the river system, and is a contributing factor to the geomorphic response within the watershed. Investigation of the longitudinal connectivity of the Watonwan watershed indicated a bridge density of 0.20/mi2, and a culvert density of 0.17/mi2. Furthermore, 11 dams are located in the Watonwan watershed; 2 of the dams are potential barriers to fish passage at most water levels. Fluvial geomorphology survey results indicated 7 of 11 survey reaches have lateral floodplain connectivity. Evidence of past channelization at or near geomorphology field sites was apparent at 10 of 11 reaches surveyed. According to the Minnesota Pollution Control Agency (MPCA), 70% of the Watonwan watershed has altered watercourses (e.g., channelized or impounded). Channelized systems have limited floodplain connectivity and are often incorrectly sized (e.g., cross sectional area to drainage area, width/depth ratios), not allowing the channel to effectively transport the sediments of its watershed. Rivers and streams in the Watonwan watershed that are maintaining deep-rooted perennial vegetated riparian corridors, and those able to access their floodplains to dissipate energy during high flows, are showing greater sign of stream stability. Overall, the objective is to improve the health of the watershed, to enhance agricultural sustainability, groundwater conservation, fish and wildlife habitat, biodiversity, recreation, and water quality throughout the watershed. As important as the restoration of disturbed areas is, focus must also be set to protect undisturbed areas generating multiple ecosystem services that appear to be near reference condition. Instability was documented at several of the survey sites"