Analysis of Shade Provided by Grassy and Woody Riparian Vegetation along a Small Coldwater Stream in the Upper Midwestern United States

Abstract

Rising water temperatures in urban and farmed watersheds threaten the survival of coldwater fish and macroinvertebrates. Baseflow stream temperatures on dry and warm summer days can be mitigated by managing groundwater discharge, in-stream morphology, and riparian vegetation. The riparian canopy intercepts and reflects incoming solar radiation with variable effectiveness dependent on canopy structure, channel width, stream orientation, and other site characteristics. While multiple benefits of both grassy and woody buffers for coldwater biota are identified in the literature, there is limited research quantifying shade provided by grassy vegetation along narrow channels, which are a common setting of headwater streams. This thesis reviews possible instruments, measures, and methods of assessing grassy riparian shade as a control for stream temperature. Hemispherical photographs (HPs) were collected and analyzed using WinSCANOPY to compare grassy and woody riparian shade along Brown’s Creek, a small coldwater stream in urban Stillwater, Minnesota impaired by high temperatures and turbidity. Grassy riparian shade ranged from 10% to 61% with an average of 34% and solitary trees increased shade above 80%. The field-based HP results for points along the stream were upscaled using a relationship developed to correct LiDAR-based shade predictions along the length of the stream. Increasing shade to at least 85% through riparian management activities reduced monthly mean stream temperatures simulated in the summer with CE-QUAL-W2 by 0.23 to 0.78°C. A simple numerical model of shade provided by grassy vegetation indicated that total daily shade between 19% and 56% could be increased up to 71% by minimizing channel width and exposed banks while also maximizing canopy height, overhang, and density. Tree plantings setback from the water’s edge could incrementally increase shade by 21% and 7% in east-west and north-south oriented channels, respectively, reaching up to 78% total daily shade. The shade model scenarios indicate that managing riparian grasses for optimized characteristics and planting trees setback from east-west oriented segments could provide approximately 80% shade or more where there’s also topographic shade. Additional thermal pollution control strategies will also be needed to support biotic health in Brown’s Creek.