BEDROCK CHANNEL RESPONSE TO VARIABILITY IN ROCK STRENGTH AND ROCK-UPLIFT RATE IN THE SANTA YNEZ MOUNTAINS, CALIFORNIA

Recent theoretical models suggest that topographic characteristics of bedrock channels reflect the interactions of rock-uplift rate, substrate resistance, and the erosive ability of the river. At present, however, these interactions are poorly calibrated. Here we investigate bedrock channels developed across the southern flank of the Santa Ynez Mountains, California, where rock strength and rock-uplift rate are well constrained. Regression analysis of river slope versus drainage area taken from digitally extracted stream profiles yields both the channel concavity as well as a channel steepness index for each stream. Field measurements of intact rock strength and channel width within select catchments complement our dataset. We find that streams flowing from resistant to less-resistant bedrock units have highly concave profiles and oversteepened lower reaches. These data imply that variability in rock strength exerts a strong influence on channel profiles in this field site. Additionally, we compared streams experiencing a rock-uplift rate of 0.75 mm/yr to channels flowing through rocks uplifting at ~5 mm/yr. This comparison was restricted to streams that erode only rocks of similar strength. Results show that a roughly 6-fold increase in rock-uplift rate induces channels ~2 times as steep and ~2.5 times as narrow as low-uplift streams. Channel observations suggest that bedrock stream width is readily adjustable, directly challenging the widespread assumption that stream gradient is the only bedrock parameter free to adjust to increases in tectonic rates. We further conclude that the combined adjustment of channel gradient and width is consistent with a model where channel incision rate is linearly proportional to mean bed shear stress.