Coevolution of Bedrock Channels and Hillslopes on a Topographically Pre-steady-state, Growing Fold

Quantitative studies of actively uplifting topography in various stages of development can yield special insights into the processes involved in landscape evolution. By exchanging space for time (the ergodic theorem) on a landform of spatially varying age and with reasonably well constrained initial conditions, one can view a landscape's development as one does in a more controlled numerical model. Wheeler Ridge, at the southern end of the San Joaquin Valley, California, is a laterally propagating anticline with a well-defined history of Quaternary uplift (Keller et al., 1998, GSA Bulletin, v. 110, p. 298-310). Nearly planar, alluvial surfaces that are preserved on the ridge project a reasonably well-dated envelope that approximates the original, pre-erosion shape of the fold. Simple morphometric and hydraulic values related to erosion processes, such as hillslope gradient and curvature, local relief, and channel shear stress, were calculated or estimated across Wheeler Ridge from a high quality and high-resolution (10 m) synthetic aperture radar digital elevation model (DEM). DEM analysis straightforwardly shows young regions of the fold with minimal uplift, gentle channel gradients, and gentle, rounded hillslopes give way to older regions with greater rock uplift, steeper channel gradients, steeper, gullied hillslopes and the presence of both shallow and bedrock landslides. These steep slopes and high relief values result from relatively rapid channel incision ( ~0.3 m/kyr), rates of which can be integrated over the age of the landform given estimates of erosion (envelope surface minus modern topography) and duration of erosion (time since fold was uplifted above base-level). The forelimb of the oldest part of the fold (Q5 in Keller et al, 1998; ~185 ka) has significantly higher channel and hillslope erosion rates than predicted by estimates of channel shear stress and measurements of hillslope curvature. This does not appear to be an artifact of the rates being measured over time-intervals of varying lengths. We also argue that climatic effects can be ruled out. Rather than being a response to base-level drop, the observed increase in channel incision rate can be explained as the consequence of more active hillslope processes and steeper hillslope gradient. These are likely to increase both the potential for runoff and the sediment that the streams must carry, which, in long-term detachment-limited channels, may be effective abrasional tools.