Sources, Transport, and Storage of Sediment at Selected Sites in the Chesapeake Bay Watershed
By Allen C. Gellis, Cliff R. Hupp, Milan J. Pavich, Jurate M. Landwehr, William S.L. Banks, Bernard E. Hubbard, Mike J. Langland, Jerry C. Ritchie1, and Joanna M. Reuter
1 U.S. Department of Agriculture, Agricultural Research Service
The Chesapeake Bay Watershed covers 165,800 square kilometers and is supplied with water and sediment from five major physiographic pro vinces: Appalachian Plateau, Blue Ridge, Coastal Plain, Piedmont, and the Valley and Ridge. Suspended-sediment loads measured in the Chesapeake Bay Watershed showed that the Piedmont Physiographic Province has the highest rates of modern (20th Century) sediment yields, measured at U.S. Geological Survey streamflow-gaging stations, and the lowest rates of background or geologic rates of erosion (~10,000 years) measured with in situ beryllium- 10. In the agricultural and urbanizing Little Conestoga Creek Watershed, a Piedmont watershed, sources of sediment using the "sediment-fingerprinting" approach showed that streambanks were the most important source (63 percent), followed by cropland (37 percent). Cesium-137 inventories, which quantify erosion rates over a 40-year period, showed average cropland erosion of 19.39 megagrams per hectare per year in the Little Conestoga Creek Watershed. If this erosion rate is extrapolated to the 13 percent of the watershed that is in cropland, then cropland could contribute almost four times the measured suspended-sediment load transported out of the watershed (27,600 megagrams per hectare per year), indicating that much of the eroded sediment is being deposited in channel and upland storage.
The Piedmont has had 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 a large percentage of sediment that was deposited in upland and channel storage, and behind thousands of mill dams. The effects of these land-use changes on erosion and sediment transport are still being observed today as stored sediment in streambanks is a source of sediment. Cropland is also an important source of sediment.
The Coastal Plain Physiographic Province has had the lowest sediment yields in the 20th Century and with sandy soils, contributes little fine-grained sediment. In the agricultural Pocomoke River Watershed, a Coastal Plain watershed, cesium-137 mass-balance results indicate that erosion and deposition are both occurring on cropland fields. Sources of sediment using the sediment-fingerprinting approach for the Pocomoke River were distributed as follows: cropland (46 percent), ditch beds (34 percent), ditch banks and streambanks (7 percent), and forest (13 percent). Cropland was a source of sediment for the two largest peak flow events, which occurred during harvesting when the ground may have been bare. The Pocomoke River Watershed is heavily ditched and channelized, conditions that are favorable for ditch bed and bank erosion. In the mixed land use (forested, agricultural, and urbanizing) Mattawoman Creek Watershed, a Coastal Plain watershed, sources of sediment using the sediment-fingerprinting approach were distributed as follows: streambanks (30 percent), forest (29 percent), construction (25 percent), and cropland (17 percent). Mattawoman Creek Watershed drains a rapidly developing region with 182 hectares (approximately 1.26 percent of the watershed) under construction. Sediment from construction sites was also determined as a source of sediment in the Mattawoman Creek Watershed. The sediment-fingerprinting source results for the three watersheds analyzed, show that in all watersheds, both the stream corridor and agriculture were significant sources of sediment. Forest as a source of sediment in the Mattawoman Creek Watershed may indicate that these forests are being disturbed and forest soils are eroding.
Bare ground can be an important sediment source. Spatial analysis of bare ground in the Little Conestoga Creek Watershed using satellite imagery between 2000 and 2005 showed that the majority of bare ground was classified as pasture. Bare ground was correlated to the growing season with the highest percentages occurring in the early spring (April, 34 percent) and after the fall harvest (December, 38 percent). The lowest percentage of bare ground (10 percent) occurred in August. Results of the sediment-fingerprinting analysis for the Pocomoke River and Mattawoman Creek Watersheds showed cropland as a source during harvesting and before planting. For the Little Conestoga Creek, however, flow may be a more important factor than seasonality in determining sediment sources.
Long-term and short-term flood-plain deposition rates Bay Watershed to determine rates of deposition. Results showed that a Piedmont stream and Western Shore Coastal Plain streams had higher sedimentation rates than Eastern Shore Delmarva Peninsula streams. The lowest deposition rates occurred on the channelized reaches of the Eastern Shore Pocomoke River. Extrapolating sedimentation rates to each study river's gross flood-plain area indicated that flood plains trap from 21 to over 100 percent of the river's sediment load. The flood plains of Coastal Plain rivers trap large amounts of sediment that otherwise would be delivered to the Chesapeake Bay.