Effects of urbanization on stream flow, sediment, and phosphorous regime

Abstract

Urbanization, if not properly managed, can lead to stream channel degradation termed Urban Stream Syndrome (USS). However, the effects of different levels of urbanization and thresholds for degradation are not well understood. The USS is characterized by increased frequency and magnitude of flood flows, increased rates of channel erosion, and degradation of aquatic habitat. In this study, changes over time in Flow Duration Curves (FDC) and Sediment Rating Curves (SRC) and resulting changes in sediment and phosphorus loads in watersheds undergoing various levels of urbanization were examined as candidate indices for detecting the presence of USS. We observed that watersheds undergoing urbanization show substantial shifts in the FDC, causing a notable increase over the whole range of stream flows and tilt in the SRC, raising the exponent and decreasing the scalar coefficient. The scalar coefficient in the SRC correlates with the percentage of the catchment area with exposed soils available for erosion, which decreases notably due to urbanization, and the exponent of the SRC increases in concert with increases in impervious cover. Changes in the exponent of the SRC associated with increased urbanization in the watershed, provide an additional diagnostic tool for identification of watercourses prone to developing urban stream syndrome. The combined effects of increased peak flows and the increase in the exponent of the SRC magnify the contribution of flood flows to the annual sediment and phosphorous loads in urbanizing watersheds. The study findings support the hypothesis that destabilized stream channels subjected to USS from increased stormflow have a significant non-point source of sediments and phosphorus from within the stream corridor in addition to off-channel upland urban area sources.