PATTERNS AND PROCESSES OF FLUVIAL INCISION IN THE HEADWATERS OF THE YELLOW RIVER, NE TIBET

The transient response of landscapes to a change in forcing can yield insight into the processes that shape the earth's surface. We use stream profile analysis to identify a transient wave of accelerated river incision that is migrating headward along the upper Yellow River, in northeastern Tibet, that is progressively excavating Tertiary sedimentary basins. Upstream of this wave of incision, low-gradient channels exhibit smooth, convex-up profiles that appear to reflect quasi-equilibrium conditions prior to the onset of rapid incision. Channel profiles steepen downstream toward the junction with the Yellow River and are associated with incised bedrock gorges and abandoned strath terraces. The knickpoints associated with the transitions between these channel forms mark the headward migration of transient incision. Knickpoints throughout the watershed are found at relatively uniform elevations, despite differences in local hydrology. We combine profile analysis with chronologic studies of Quaternary stratigraphy in the basins, fluvial terraces along the channels, and recent erosion rates inferred from 10Be inventories in modern sediment. Together, these analyses reveal low denudation rates (30-50 m/Myr) along low-gradient portions of the channel network above knick points and rapid incision (1-2 mm/yr) along the steep channels downstream. Moreover, cosmogenic burial ages from the Tongde basin, combined with OSL and radiocarbon ages from fluvial terraces, constrain the onset of incision and the rate at which the incisional wave migrates upstream. Finally, multiple terrace levels near the town of Jinguum indicate that incision rates rapidly reach a maximum following the arrival of the transient wave, and then progressively decrease with time. We synthesize these data and place them into an interpretative framework using simple 1-D simulations of channel profile evolution. Using a stream-power form of a channel evolution equation requires that the slope exponent (n) be less than unity to adequately explain channel profile form. Moreover, our results indicate that the coefficient that describes erosive efficacy (K) must change across the knickpoints. We are currently exploring models that incorporate sediment flux to see if this behavior reflects an influence of downstream changes in sediment load.