Comparing gravel-bed rivers in paired urban and rural catchments of southeastern Pennsylvania

Surveys in eight paired urban and rural watersheds illustrate how urbanizat ion changes fluvial morphology and processes. Our data also provide quantit ative criteria for evaluating stream-restoration projects in urban areas. B ankfull depth, reach-averaged bed slope, and median grain size are similar in urban and rural watersheds. The median width of urban channels is 26% la rger than the median width of rural channels. The median sinuosity is 8% lo wer in urban channels and pools are 31% shallower, The median composite Man ning's n based on median grain diameter, pool depth, and channel sinuosity is 10% lower in urban streams, while the median bankfull discharge per unit drainage basin area is 131% higher in urban channels. Histograms of bed se diment-size distributions in urban channels lack a secondary mode in the si ze range 2-64 mm characteristic of rural channels, indicating that these si zes tend to be selectively removed from urban channels. However, bankfull S hields stresses in urban and rural channels exceed typical threshold values at most sites, indicating significant bedload transport at bankfull stage. Apparently, increased peak discharges caused by decades of urbanization ha ve not removed all the transportable sediment from these urban stream chann els. We speculate that the supply of sediment to urban channels from hillsl ope processes and channel erosion remains significant, even though much of the upland surfaces of these urban catchments are covered with nonerodible impervious surfaces.