SPATIALLY AVERAGED SEDIMENT TCN CONCENTRATIONS DESCRIBE STEADY-STATE AND TRANSIENT LANDSCAPE PROCESS IN NE TIBET

Observations of river incision are historically the most readily available and widely used measures of landscape erosion rates. However, additional observations must be made about the behavior of the hillslope regime in order to fully describe the behavior of both the fluvial and the spatially dominant non-fluvial portions of a landscape. Spatially averaged catchment denudation rates determined from the terrestrial cosmogenic nuclide (TCN) content of fluvial sediments represents a single, convolved measure of both fluvial and hillslope erosion rates and thus hold great promise for quantitative geomorphic studies. We compare fluvial channel gradients and latest Pleistocene-Holocene incision rates to denudation rates determined from stream sediment TCN content within the headwaters of the Yellow River, northeastern Tibetan Plateau. In near steady-state portions of the landscape and at relatively low incision rates, basin-wide denudation rates of 0.28-0.35 mm/yr are equivalent to fluvial incision rates of adjacent channels of ~0.3 mm/yr. A wave of increased fluvial incision, marked by a two to three fold increase in channel gradient index and incision rate, is interpreted to be currently propagating headward along the Yellow River and tributaries into the northeastern Plateau. In contrast to the steady state setting, channel incision rates of >1mm/yr within this transient wave are observed adjacent to only moderately higher basin denudation rates of ~0.4 mm/yr. Hillslopes adjacent to the transient fluvial incision show a planimetric dominance of lower gradients with higher values confined to near incised channels. These results suggest that the TCN technique is faithfully recording average catchment denudation rates in a near steady-state setting and highlights the time lag between fluvial and hillslope response to a transient change in river incision rate.