HR: 08:00h
AN: H51H-01    [Abstracts]
TI: Coalescing Debris-Fill Complexes in Headwater Valleys of the Oregon Coast Range
AU: * Lancaster, S T
EM: lancasts@geo.oregonstate.edu
AF: Dept. Geosciences, Oregon State University, 104 Wilkinson Hall, Corvallis, OR 97331-5506 United States
AU: Grant, G E
EM: Gordon.Grant@oregonstate.edu
AF: PNW Research Station, USDA Forest Service, Forestry Sciences Laboratory, Corvallis, OR 97331 United States
AB: Headwater streams are important links in the sediment transport chain connecting production from steep uplands to supply in downstream, fish-bearing channels. In headwater basins that are forested and where debris flows are common, debris-flow deposition forms valley-spanning dams of wood, boulders, or both, and significant volumes of sediment are stored behind these dams. In three headwater streams (basin areas of approximately 2 sq. km) in the Oregon Coast Range, the following data were collected: (a) longitudinal channel profiles; (b) locations and heights of debris dams; (c) sediment storage volumes; and (d) exposed bank stratigraphy. In all three basins, these data indicate a zone of concentrated mainstem debris-flow deposition where stream gradients first decrease to approximately 10%. Deposits in this zone do not form fans per se but, rather, comprise coalescing debris-fill complexes. These complexes have large unit storage volumes (defined as valley-floor sediment storage volume per upstream contributing area), and because coalescing debris-fill complexes are formed in headwater valleys with relatively low bankfull discharge, these complexes represent a buffer between episodic sediment input from the hillslopes and continual sediment output to fish-bearing streams. An empirical model of susceptibility to debris-flow inundation is based on hypothesized landslide susceptibility, an observed relationship between debris-flow runout length and elevation change between source and deposit, and tributary junction angles and corroborates the occurrence of coalescing debris-fill complexes where greater debris-flow susceptibility arises and is attributable to multiple sources, e.g., tributaries and main stem. Field data and simulation modeling indicate that debris-flow deposition in these complexes is sensitive to wood volume and depositional history: Entraining wood from valley floors and spreading over prior deposits decrease debris-flow velocities and favor deposition. Depletion of large, entrainable wood upstream of these complexes might lead to debris flows bypassing this zone and traveling through reaches where deposition on prior deposits is less likely. Conversely, present debris-flow deposition in reaches downstream of this zone may be less sensitive to changes in entrainable wood volume and depositional history because of overriding controls associated with tributary junctions, e.g., abrupt changes in stream gradient, flow direction, and valley width.
DE: 1803 Anthropogenic effects (4802, 4902)
DE: 1804 Catchment
DE: 1810 Debris flow and landslides
DE: 1825 Geomorphology: fluvial (1625)
DE: 1861 Sedimentation (4863)
SC: Hydrology [H]
MN: Fall Meeting 2005