T123 Sediment in Fluvial Systems: Production, Transport, and Storage at the Watershed Scale I

Sunday, 5 October 2008
8:00 AM - 12:00 PM

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

Paper number: T123-4
Program: Technical Sessions
Time: 8:35 AM – 8:50 AM

Authors

Abstract

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.