Drainage density on progressively tilted surfaces with different gradients, Wheeler Ridge, California

Wheeler Ridge in the Southern San Joaquin Valley, California, is an anticlinal fold which has been progressively uplifted during the last 250 ka. Drainage networks on the ridge become younger as the anticline's eastern tip is approached. Because of the fold's asymmetric shape, surfaces on opposite flanks of the ridge have similar ages but very different gradients. The ridge provides important insights into drainage development on progressively tilted surfaces, as existing studies are restricted to static topography. A surface gradient of between 4·8° and c .10° is needed to initiate channel networks. This gradient threshold is consistent with previous studies of the gradient and upslope area needed to incise a channel through overland flow. Comparison of coeval drainage networks on opposite flanks of the ridge allows the controversial relationship between drainage density and gradient to be investigated. A lower valley density is observed on the higher gradient flank of Wheeler Ridge. Field observations from the ridge indicate that this inverse relationship is associated with hillslope erosion by shallow mass‐wasting, the rate of which increases rapidly as a threshold gradient is approached. Comparison of data from Wheeler Ridge with other field studies and numerical models, shows that the form of the relationship between gradient and drainage density is process‐dependent. A positive correlation occurs when erosion is a result of overland flow, whilst a negative correlation occurs where erosion is dominated by shallow mass‐wasting. Copyright © 1999 John Wiley & Sons, Ltd.