SEDIMENT PRODUCTION AND STORAGE ALONG MID-ATLANTIC UPLAND STREAMS

Sediment budgets and other geomorphic investigations support a conceptual model of sediment production and storage along mid-Atlantic streams with well-developed floodplains. Frequent bedrock exposures reduce the rate and extent of lateral and vertical channel migration, thus limiting the channel's ability to respond to changes in the supply of water and sediment. Trees also limit rates of bank erosion, even where riparian forests are poorly developed, further restricting rates of channel changes. For example, comparison of aerial photographs from 1937 to 2005 over 25 miles along the South River, Virginia, a steeply sloping gravel-bedded channel, indicates that rates of lateral migration are so low as to be barely resolvable along much of the channel. Historical events, particularly European settlement and the subsequent the rise and decline of agriculture, have played a well-documented role in determining present patterns of sedimentation. Currently, eroding channel boundaries are a significant source of sediment. In Montgomery County, Maryland, erosion of the channel bed and banks accounted for 30-40% of the total sediment yield from 1952-1996 (1/2 of this material represents erosion of post-settlement “legacy” deposits). Floodplain construction is limited to overbank deposition on narrow levees (levee sediment storage in Montgomery County is equivalent to approximately 15-49% of annual average sediment yield, while levee storage along South River is considerably less than this amount) and minor amounts of episodic deposition (and erosion) of sand and gravel on bar surfaces. Channels appear to be only partly free to adjust to changes in sediment supply and discharge, hence are best described as “bedrock-alluvial” channels rather than as fully “alluvial channels”. Cohesive post-settlement floodplains are eroding very slowly, and will ultimately be replaced by thin deposits of sand and gravel during coming centuries. Given their complex history and ongoing evolution, it is difficult to apply concepts of “fluvial equilibrium” and “grade” to these channels.