Debris dams and the relief of headwater streams

Abstract: In forested, mountain landscapes where debris flows are common, their deposition commonly forms valley-spanning dams of wood, boulders, or complex mixtures of both in headwater valleys. Sediment impoundment behind these dams causes alluviation in what would otherwise be bedrock channels. In this paper, the effects of debris dams on the evolution of headwater valley profiles over geologic time are examined. In the Oregon Coast Range, USA, longitudinal profiles of three headwater channels (approximately 2 km2 maximum contributing area), two of which are nominally bedrock, and all debris dams were surveyed. Channel and valley widths were measured, and surface bed material measurements (pebble counts) were taken at several locations along one stream. Cumulative relief in debris dams is highly variable within and among streams, reaching a maximum of 58% of profile relief at one location. The surveyed dams comprise 9.8%, 19%, and 6.4% of total basin relief in the three corresponding drainage basins, respectively. A model of bedrock erosion and channel profile evolution is derived that accounts for (a) fractional coverage by the channel of total valley width at any given time and (b) shielding by sediment impounded behind debris dams. The equation for incision rate, assumed constant, is solved for valley gradient as a function of contributing area, and parameter values are estimated from or supplied by field data. Results are compared to stream-gradient–contributing-area relationships obtained for the field sites. These comparisons suggest 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. Longitudinal profiles with and without the effects of debris dams are modeled for one forced-alluvial, i.e., nominally bedrock, channel. For these model profiles, which extend only as far as the surveyed profile at this site, 55% of the profile relief is due to debris dams and the concomitant shielding of the bed by impounded sediment. The 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. [Copyright &y& Elsevier]