Forty-three US Geological Survey gauges with records greater than 15 yrs located in watersheds less than similar to 250 km(2) were used to model the effects of suburbanization on streams in semi-arid southern California. The watersheds spanned a gradient of urban development, ranging 0-23% total impervious area in 2001. With little flow control at the subdivision scale, most impervious area in the region is relatively well-connected to surface-drainage networks and hydrologically effective. Consequently, total impervious area was an effective hydrologic surrogate for urbanization, emerging from an expansive array of geospatially-derived hydrologic variables as a statistically-significant (p < 0.05) predictor of instantaneous peak-flow rates at the 1.5- and 2-yr recurrence intervals and the durations of all geomorphically-important flows. To represent the effects of urbanization on flow durations, we developed duration density functions by using power functions (typical R-2 > 0.95) to predict occurrence of logarithmically-binned mean daily discharges greater than some nominal value. This approach expands on previous scaling procedures to produce histogram-style cumulative flow durations for ungauged sites using urbanization extent and other watershed descriptors. For a particular watershed size and climatic setting, urbanization resulted in proportionally-longer durations of all geomorphically-effective flows, with a more pronounced effect on the durations of moderate flows. For example, a representative watershed with similar to 20% imperviousness could experience five times as many days of mean daily flows on the order of 100 cfs (3 m(3)/s) and approximately three times as many days on the order of 1000 cfs (30 m(3)/s) relative to the undeveloped setting. Increased duration of sediment-transporting flows is a primary driver of accelerated changes in channel form that are often concurrent with urbanization throughout southern California, particularly in unconfined, fine-grained geomorphic settings. We did not have comparable studies on flow durations from other regions; however, the peak factors presented herein (e.g., sixfold increase in Q(2) at 20% imperviousness) are greater than studies from humid temperate regions suggesting that semi-arid regimes may be more susceptible to urbanization than other climatic settings. (C) 2011 Elsevier By. All rights reserved.