Controls on the Mean Spacing of Channels on Incised Alluvial Terraces in the Western U.S.

Abstract:

Incised alluvial terraces with a relatively uniform channel spacing are commonly found throughout the western United States. The characteristic spacing of low-order channels on these surfaces generally increases as the height of the surface increases with respect to base level. We measure the characteristic spacing of channels in parallel drainage networks on incised alluvial terraces and unconsolidated fill deposits in several locations throughout the western U.S. where incision is the result of a quantifiable change in base-level elevation. Mean channel spacing on terraces subjected to small amounts of base-level drop (e.g 20 m) can be more than three times smaller than that typically observed on terraces subjected to larger amounts (e.g. 100 m) of base-level drop. The relationship between the magnitude of base-level drop and mean channel spacing is further examined by using a numerical model. Numerical experiments suggest that the observed relationship between relief and channel spacing is the result of feedbacks between the depth of channel incision, mass wasting, and nonlinear increases in the rate of colluvial sediment transport on steep hillslopes that lead to valley widening. In extremely high relief cases, valley side slopes may lengthen sufficiently to support the formation of first order, tributary valleys, leading to the development of a trellis-like drainage system. The rapid widening of valley side-slopes can prevent the growth of adjacent, ephemeral channels and promote a greater mean channel spacing. Model results offer additional insight into the response of landscapes to rapid base-level fall and provide a preliminary basis for understanding how various amounts of base-level drop influence channel network morphology.