RAPID RIVER INCISION AND KNICKPOINT MIGRATION INFERRED FROM TERRACES DATED USING 10BE, JAMES RIVER, CENTRAL VIRGINIA, USA

We present dates on fluvial terraces obtained from in situ-produced 10Be that suggest rapid river incision along the James River, central Virginia Piedmont. Dates on two terrace surfaces at one location suggest incision rates of ~55-125 m/Myr averaged over the last ~1 Myr. These rates exceed estimated Piedmont interfluve lowering rates of ~20 m/Myr (Pavich, 1989). Moreover, they exceed even greater denudation rates generally found upstream in the valley and ridge. 10Be dates (currently in process) of three additional terrace locations will help to further refine incision rates over a ~50 km reach. If the additional dates yield similar incision rates, the difference between interfluve and river lowering rates suggests a disequilibrium Piedmont landscape with growing topographic relief rather than one in dynamic equilibrium. Entrenchment of the James River and major tributaries below a gently rolling Piedmont upland may be the topographic expression of this disequilibrium. Migration of knickzones observed on the James River and tributary longitudinal profiles may be responsible for the rapid fluvial incision. Mapped terrace profiles will be used to test the migratory nature of one major (15 m) James River Piedmont knickzone now ~100 km upstream of the Fall Zone.