EFFECT OF IN-STREAM GEOMORPHIC STRUCTURES ON HYPORHEIC FLOWPATHS AND TEMPERATURES

In-stream geomorphic structures such as debris dams and log dams are common in undisturbed streams, are often installed as part of stream restoration projects, and can significantly enhance hyporheic exchange. This exchange is important ecologically for many reasons, including potentially altering summer temperature patterns in streams. We conducted experiments that evaluated the impact of structure presence and size using a variable-height weir-type structure installed in a stream. The thermal and hydraulic response of the hyporheic zone to the structure was monitored in detail with a three-dimensional temperature and water level sensor network. In addition, novel tracer release and monitoring methods were developed to characterize structure-induced hyporheic flowpaths in spatially explicit ways. Results indicate that structures induce a coherent hyporheic flow cell that varies predictably with structure height, but whose flowpaths vary spatially due to substrate heterogeneity. This structure-induced hyporheic flow cell develops a characteristic thermal profile both vertically and horizontally that reflects overall hydraulic forcing of the structure but also flowpath heterogeneity.