Effects of urban development in the Puget Lowland, Washington, on interannual streamflow patterns: Consequences for channel form and streambed disturbance

Recovery and protection of streams in urban areas depend on a comprehensive understanding of how human activities affect stream ecosystems. The hydrologic effects of urban development and the consequences for stream channel form and streambed stability were examined in 16 streams in the Puget Lowland, Washington, using three streamflow metrics that integrate storm-scale effects of urban development over annual to decadal timescales: the fraction of time that streamflow exceeds the mean streamflow (T(Qmean)), the coefficient of variation of annual maximum streamflow (CV(AMF)), and the fraction of time that streamflow exceeds the 0.5-year flood (T(0.5)). Urban streams had low interannual variability in annual maximum streamflow and brief duration of frequent high flows, as indicated by significant correlations between road density and both CV(AMF) and T(0.5). The broader distribution of streamflow indicated by T(Qmean) may be affected by urban development, but differences in T(Qmean) between streams are also likely a result of other physiographic factors. The increase in the magnitude of frequent high flows due to urban development but not their cumulative duration has important consequences for channel form and bed stability in gravel bed streams because geomorphic equilibrium depends on moderate duration streamflow (e.g., exceeded 10% of the time). Streams with low values of T(Qmean) and T(0.5) are narrower than expected from hydraulic geometry. Dimensionless boundary shear stress (tau*) for the 0.5-year flood was inversely related to T(0.5) among the streams, indicating frequent and extensive bed disturbance in streams with low values of T(0.5). Although stream channels expand and the size of bed material increases in response to urban streamflow patterns, these adjustments may be insufficient to reestablish the disturbance regime in urban streams because of the differential increase in the magnitude of frequent high flows causing disturbance relative to any changes in longer duration, moderate flows that establish a stable channel.