Island formation through bar growth and floodplain incision in the bedrock controlled South River, Virginia

Date
2012
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University of Delaware
Abstract
Islands in braided and meandering alluvial channels form by bar accretion and floodplain incision, however, island formation in bedrock-controlled channels is poorly understood. The South River is a single-thread, sinuous, gravel-bed, bedrock river. It is neither meandering nor braided but aerial photographs show the development of gravel bars and the formation of islands that have formed through floodplain incision. This study deciphers processes that lead to both types of island formation and their role in the channel morphology of the South River. The South River was analyzed using aerial photographs and work in the field provided additional data that were used to identify islands and their properties. A Geographic Information System (GIS) was used to evaluate historical aerial photographs dating back to 1937 for location, morphology, origin, and development of islands along a 39 km study reach. Field studies included the surveying of cross sections to determine elevations of islands relative to neighboring floodplains, as well as pebble counts and cores to define sediment characteristics. Aerial photographs indicate that nine islands had formed before and an additional 11 islands formed after 1937, placing the average island formation frequency at 0.008 islands per km per year since 1937. Field data indicate that elevation, grain size, stratigraphy, and vegetation of 17 islands closely resemble those of the floodplains, supporting the hypothesis that those islands formed through floodplain incision. The sediment of three islands was similar to that of the channel and did not show similarities to floodplains or any other islands indicating formation through gravel bar deposition. Studies of bank erosion rates along the South River demonstrate that 33% of bank erosion along the South River occurs in divided reaches of the channel associated with islands. Understanding the formation and evolution of these islands may allow for an accurate prediction of future island development and possible sediment sources.
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