A method for advancing understanding of streamflow and geomorphological characteristics in mixed-land-use watersheds

Methods are needed to quantify stream geomorphological response to land use and hydroclimatic variability. The method applied herein incorporated channel measurements from a physical habitat assessment (channel width, bankfull width, thalweg depth, and estimated cross-sectional area), and streamflow data collected via an experimental watershed study, to identify factors contributing to longitudinal variation in stream morphology in a mixed-land-use watershed of the central U.S. Channel and bankfull width ranged from 0.8 m and 1.8 m, respectively, at the headwaters, to 70 m and 74 m, respectively, mid-watershed. Minimum thalweg depth (0.2 m) was observed at the headwaters, while the maximum (8.6 m) was observed at the mouth. Mann Kendall results indicated a significant positive trend (p < 0.001) for each of the three metrics over the entire length of the stream. However, smaller sections of the creek exhibited contrasting trends consistent with channel widening and incision. Cross-sectional area significantly (p < 0.001) increased from the headwaters to the mouth. However, two reaches exhibited drastic reductions in cross-sectional area, which could indicate reduced channel capacity and localized flood hazard. The longitudinal pattern of channel width, bankfull width, and cross-sectional area showed the strongest (R² > 0.7), significant (p < 0.05) correlations with the estimated longitudinal pattern of 99th percentile flows, while thalweg depth correlated most strongly with 75th percentile flows (R² = 0.77, p < 0.001). Collectively, results emphasize the importance of high flows to channel morphology, but identify other factors (e.g. land use, geology, physiography) that variously contribute to observed stream geomorphology. Furthermore, results demonstrate the capacity of the method to provide detailed, quantitative characterizations of physical and hydrologic features, and to identify potential drivers of channel morphology in contemporary mixed-land-use watersheds.