SEDIMENT PRODUCTION IN AN URBANIZING WATERSHED

Between 1951 and 1996, the percentage of the Good Hope tributary watershed covered with impervious surfaces increased from 1.4% to 7.5%. Using surveyed cross-sections and regression equations relating drainage basin area and imperviousness to channel area, we estimate that the channel area increased by a factor of 1.7 during the same period. We infer changes in sediment production and storage related to channel enlargement (exclusive of upland sediment yield, which we did not measure). Enlargement of first-order streams tributary to the Good Hope tributary supplied 3200 (+/- 1100) m3 to the main channel, while enlargement of the Good Hope tributary itself produced an additional 3200 (+/- 600) m3. Sediment is stored on the valley flat that defines most of the valley floor, and also in small lateral accretion floodplains that are inset into the older floodplain deposits of the valley flat. Using dendrochronology to estimate deposition rates on the valley flat, we determine that 4,000 (+/- 1900) m3 was stored on the valley flat from 1951-1996. By solving a mass balance equation, we estimate the sediment production associated with channel enlargement from 1951-1996 to be 2,400 (+/- 2,200) m3, equivalent to 10.7 t/km2/yr. These results indicate that channel enlargement in urbanizing Piedmont watersheds produces relatively little sediment compared to many other geomorphic settings. Since 1997, the Montgomery County EPA has monitored water quality at the mouth of one of the first-order tributaries to the Good Hope. This data indicates that roughly 95.2 tonnes (37 m3) of suspended solids are transported past the sampling station each year, and that roughly 349.3 tonnes (135 m3) of suspended material was removed between April 1, 1997 and December 31, 2000. An assessment of the magnitude of the change has been made by comparing observations and photographs of streams from the Paint Branch watershed taken in 1949 with recent data and images.