EROSION BY KNICKPOINT RETREAT IN THE UPPER JAMES RIVER BASIN

We assemble data from river profiles, terrace distribution, soil characteristics, and valley morphology that point to erosion by the passage of knickpoints as a significant if not dominant component of denudation. The rivers of the upper James basin have longitudinal profiles containing pronounced convexities, not exclusively associated with rock type, and incised channels below broad uplands. The uppermost of the convexities or knickzones define the break between broad and incised valleys. Above the knickzone in the Maury River subcatchment, floodplains and low terraces are large, while below the knickzone a 60-80 deep incised valley with few terraces is locally flanked by large terraces at the upper margins of the incised valley. This transition is confounded somewhat by the change of lithology through the knickzone from Devonian shales to Ordovician carbonates. However, these same rocks along the James River do not present the same changes of valley shape; valley shape does change at other reaches not controlled by rock type along the James, Jackson and Cowpasture Rivers, but they are less distinct knickzones. The valley immediately below knickzones is inundated with sediment, which is stored in large floodplains and low terraces. The along-profile change of elevation versus distance of these largest terraces define the rate of incision compared to rate of retreat. In softer rocks, the terraces form what appears to be a correlative level, but the large terraces descend toward the channel elevation in harder rocks as the upstream pace of retreat slows. Two and perhaps three separate knickpoints are mapped from the profile shape of tributary streams below the anomalously steep and high Goshen Pass knickzone on the Maury River by using deviation from the average slope-area relationship. Alternating bedrock-floored and slab and sediment rich reaches of the Maury River in Goshen Pass suggest that these knickzones have collapsed into one short reach through resistant rocks.