Regional impacts of urbanization on stream channel geometry: Importance of watershed area and channel particle size

Urbanization increases impervious surface cover, expands the drainage network, and increases flow velocity, which increases storm runoff, peak discharges and rates of stream channel erosion. Southern California has experienced rapid urbanization over the past several decades and has the potential for stream channel degradation. San Diego County has implemented a Hydromodification Management Plan to protect channels from erosion through monitoring new development projects and their increases in runoff, but no studies have identified the dominant controls and the impact of urbanization on channel geometry. A synoptic survey of 80 field sites by the California Environmental Data Exchange Network (CEDEN) and additional field surveys from 2013-2014 were used to develop regional curves relating bankfull cross sectional area (Axs), width (w), mean depth (d), and discharge (Qbf) to watershed area (Aw ). Regional curves were compared for urban and reference sites and compared to other regional curves developed for southern California. Multiple regression models were used to identify dominant watershed and channel controls on geometry, including Aw, percent impervious cover (I%), mean annual precipitation, underlying geology, longitudinal slope, and hydrologic soil group. For the reference streams, regional curves were most robust for w (p < 0.05) and Axs (p < 0.05). The regional curves for urban streams had substantially larger coefficients in the regional curves, indicating that urban channels have larger w and Axs for a given watershed size. The most predictive variables for w were Aw and I%. Aw is a predictor of Axs only for reference sites; when all sites are included in a regression model, I% was the only predictive variable for all channel metrics, suggesting that urban-induced enlargement in smaller urban channels disrupted the natural Axs-Aw relationship. A majority (68%) of the urban channels were enlarged, defined as a Axs larger than the upper 95% confidence interval of the regional curve for reference sites. Of the enlarged channels, 73 percent were located in small watersheds (<10 km2). Channel response differed by channel substrate. Sand-bedded channels incised, while gravel-bedded channels widened. Sand-bedded channels in small urban watersheds (10 km2). Channels draining larger urban watersheds may be less susceptible to enlargement due to stabilization of channel banks through the establishment of riparian vegetation from increased urban baseflow. Management should focus on monitoring sand-bedded channels in watersheds smaller than 10 km2.