Debris Dams, Sediment Impoundment, and the Relief of Headwater Streams

In forested, mountain landscapes where debris flows are common, valley-spanning debris dams formed by debris-flow deposition are a common feature of headwater valleys. In this paper, the effects of debris dams on the evolution of headwater valley profiles over geologic time are examined. The main channels of three small (approximately 2 sq. km) watersheds in the Oregon Coast Range, USA, were surveyed, with special care given to capturing the relief of each significant step and noting the step-forming agent, whether wood, boulders, a complex mixture of the two, or bedrock (although the last is not treated herein). Channel and valley widths were also measured, and surface bed material measurements (pebble counts) were taken at one of the sites. The amount of relief in wood and/or boulder steps is highly variable within and among sites, reaching a maximum of 58% at one location. The surveyed steps comprise 9.8%, 19%, and 6.4% of total basin relief in the three sites, respectively. A model of valley profile evolution is derived that accounts for the facts that (a) only a fraction of the valley width is occupied by the channel at any given time and (b) bedrock is often shielded from erosion by sediment impounded behind debris dams. Under the assumption of steady-state incision, the equation for incision rate is solved for valley gradient as a function of contributing area, parameter values are estimated or supplied by field data, and the results are compared to plots of gradient vs. contributing area for the field sites. These comparisons suggest--but do not demonstrate--a strong effect of network structure, which varies significantly among the sites, on profile shape and relief because of the different susceptibilities to debris-flow deposition and, therefore, debris-dam formation. Specifically, in a reach where the ratio of step height to spacing drops abruptly, stream gradient also falls more steeply in a way consistent with model predictions. Finally, for one site, steady-state model profiles with and without the effects of steps are constructed. For these model profiles, which only extend as far as the surveyed profile at that site, 55% of the profile relief is due to steps and the concomitant shielding of the bed by impounded sediment. These results suggest that a significant fraction of the relief of such forested, mountain landscapes is due to the effects of relatively immobile wood and boulders deposited in the valleys by debris flows.