DRAINAGE BASIN INCISION BY KNICKPOINT RETREAT ACROSS VARIABLE ROCK TYPES, CENTRAL APPALACHIANS, VIRGINIA

The upper James River basin in the central Appalachians is incising faster than nearby Valley and Ridge drainage basins, likely begun by drainage capture in the Blue Ridge along the Blue Ridge Escarpment. The rough topography includes incised valleys in limestone uplands, high-elevation low-relief surfaces, and knickpoints/knickzones in resistant and nonresistant rocks. Mainstream and tributary profiles are irregular and stepped. Some have obvious floodplain abandonment and strath terrace formation at the heads of bedrock knickpoints. Although stream knickpoints can be compared on log-distance-downstream plots (after Hack), the ability to correlate tributary knickpoints with those on the confluent stream is limited. Automated mapping of the entire drainage network using deviation from a predicted area-slope relationship identifies diffuse waves of incision that connect steep stream segments with sometimes distant, steep valley sidewalls. However, the highly irregular and strongly positive error of NED DEM data calls for significant smoothing. Because longer averaging on stream segments eliminates lesser knickpoints found in field stream profiles, we employ a smoothing method that cancels positive error and computes smoothing in a smaller neighborhood. For the largest streams, NED DEM data incorrectly code stream elevation, and SRTM DEM data are too noisy to build stream slope at the scale of single knickpoints. Combined field and GIS mapping reveal significant geographic variation in knickzone position as measured from the basin outlet, likely caused by bedrock resistance and alignment of streams to structural orientation. Streams that cut across folded sedimentary rocks can have steeper knickpoints closer to the confluent stream than those flowing along strike. The 60 to 80-m high knickzone on the Maury River known as Goshen Pass is especially steep due to multiply duplicated sections of resistant sandstone. However, increased energy loss begins after the stream meets the resistant units and is greater per unit thickness of resistant unit than similar gaps. This morphology combined with GPS and DEM profiles of other rivers in the same unit suggests that one or more transitory knickpoints are temporarily stalled in the Pass where migration rates slow.