The influence of convex-bank floodplains on stream channel processes and morphology in the mid-Atlantic Piedmont

In low-order mid-Atlantic Piedmont streams, convex-bank floodplains evolve from bank adjacent bars. Riparian vegetation has a dramatic impact on the dimensions of these floodplains and the geometry of these channels. Convex-bank floodplains appear as a result of a surplus of coarse sediment in river channels, possibly due to land use changes upstream, and can influence sediment yield from small Piedmont watersheds.; Maps and sediment cores from convex-bank floodplains along a gravel-bedded stream in northeastern Delaware suggest that these floodplains evolve from point bars. Roughness associated with point bars reduces the flow velocity, inducing further deposition, and trapping previously deposited sediment. Upon reaching some critical thickness, the floodplains accrete laterally from the deposition of bed material along the channel's proximal edge, while they accrete vertically through over-bank sedimentation.; Data from streams in southeastern Pennsylvania explains differences in channel morphology between streams with different riparian vegetation. Rates of deposition and lateral migration are higher in non-forested reaches due to the abundance of grassy vegetation on floodplain surfaces. As a result of the constriction of the channel, near-bank velocity is higher in the non-forested reaches, causing elevated rates of cut-bank erosion. Thus, bank erodibility and floodplain depositional processes must be considered together to explain the morphology of laterally migrating alluvial channels.; Convex-bank floodplains influence the downstream sediment yield from urbanizing streams. Using the ergodic assumption, channel surveys and regression equations, drainage basin area and basin imperviousness are related to channel size. Channel cross-sectional area in the Good Hope Tributary watershed may have increased by a factor of 1.7 between 1951 and 1996 as a result of suburban development within the watershed. Channel enlargement produced 6400 (+/-1700) m3 of sediment. This sediment was stored on the valley flat and in convex-bank floodplains within the channel. Using dendrochronology to estimate deposition rates on these surfaces, it was determined that 4,000 (+/- 1900) m3 of sediment was stored on the valley flat from 1951--1996. Using a mass balance equation, it is estimated that the sediment production associated with upland development and channel enlargement from 1951--1996 is roughly 8,120 (+/- 2,200) m3, equivalent to 135 t/km2/yr.