WIDTH ADJUSTMENT OF LATERALLY MIGRATING MIXED LOAD GRAVEL-BED RIVERS OF THE MID-ATLANTIC PIEDMONT

Twenty-five years of observations of monumented cross-sections along a small stream near Carlisle, PA and soil borings demonstrate how cutbank erosion and deposition on lateral accretion floodplains create a stable width. As the channel begins to migrate laterally, floodplain deposits on the inside bank initially consist of a gently sloping sandy point bar. Through time, the inclination of the inside bank increases, and sandy deposits are replaced by overbank deposits averaging 70% mud and 30% sand. Width adjustment thus reflects a balance between processes of floodplain deposition, largely from suspension, on the inside bank, and processes of cutbank erosion on the outside bank. Observations of paired forested and non-forested reaches indicate that width adjustment processes are significantly influenced by riparian vegetation. Channels in non-forested reaches are significantly narrower than channels in forested reaches. Deposition rates on lateral accretion floodplains, measured using dendrochronology are 7 times higher in non-forested reaches than in forested reaches. Cut-banks erode 6 times faster in non-forested reaches than in forested reaches. The roughness of lateral accretion floodplains, parameterized by the board test, is higher by a factor of 5 in non-forested reaches. These data suggest that non-forested reaches are relatively narrow because dense vegetation leads to rates of deposition on the inside banks that are high enough to overcome high rates of cutbank erosion.