Sources, Transport, and Storage of Sediments at Selected Sites in the Chesapeake Bay Watershed

The sources, transport, and storage of sediments were assessed in Chesapeake Bay Watershed (165,800 km²). In the Little Conestoga Creek Watershed (110 km²), which drains agricultural and urban areas in the Piedmont, sediment sources using “sediment fingerprinting” were: streambanks (63%) and cropland (37 %). Cropland erosion rates in the Little Conestoga Creek watershed were 19.39 Mg/ha/yr based on ¹³⁷Cs inventories,. In the agricultural Coastal Plain Pocomoke River watershed (157 km²), sediment sources were: cropland (46%), ditch beds (34%), forest (13%), and ditch banks and streambanks (7%). The Pocomoke River watershed is heavily ditched and channelized. In the Mattawoman Creek watershed (134 km²), a forested, agricultural, and urbanizing Coastal Plain watershed, sediment sources were streambanks (30%), forest (29%), construction (25%), and cropland (17%). The Mattawoman Creek Watershed has 1.26% of its watershed in construction.

The Piedmont has the highest 20th century sediment yields measured at river stations and the lowest geologic erosion rates (~10,000 years) measured with in situ ¹⁰Be. The SPARROW regression model also showed the Piedmont as the highest sediment-yielding region. The Piedmont has centuries of land-use change, from forest to agriculture, to suburban and urban areas, and in some areas, back to forest. These land-use changes mobilized sediment that was deposited in uplands and channels, and behind thousands of mill dams. Long-term and short-term flood-plain deposition rates measured at selected sites in the Chesapeake Bay Watershed indicate that floodplains may trap from 21% to over 100 % of the river's sediment load.