HR: 14:10h
AN: H43H-03    [Abstracts]
TI: Sediment Transfer and Storage in Headwater Basins of the Oregon Coast Range: Transit Times from Radiocarbon-Dated Deposits
AU: Underwood, E F
EM: underwoe@geo.oregonstate.edu
AF: Dept. Geosciences, Oregon State University, 104 Wilkinson Hall, Corvallis, OR 97331- 5506,
AU: * Lancaster, S T
EM: lancasts@geo.oregonstate.edu
AF: Dept. Geosciences, Oregon State University, 104 Wilkinson Hall, Corvallis, OR 97331- 5506,
AB: Many hillslopes in mountain landscapes such as the Oregon Coast Range (OCR) are coupled to channels through episodic delivery of sediment by debris flows to valley-bottom storage and gradual evacuation by fluvial processes. We characterized transit times through storage at two tributary confluences with the mainstems of Cedar and Golden Ridge Creeks in the central OCR with dense sampling of bank exposures: 68 radiocarbon age estimates from randomly assigned locations in the tributary and mainstem channel banks provided transit-time proxies. The inferred mean transit times were 1240 ¹⁴C yrs for a debris fan tributary to Cedar Creek and 1510 ¹⁴C yrs for fluvial terraces tributary to Golden Ridge Creek. Inferred transit-time distributions have double-exponential shapes for both tributary deposits and imply that younger deposits are preferentially evacuated: while most material moves through these storage sites rapidly (with characteristic timescales of 250 and 500 ¹⁴C yrs for Cedar and Golden Ridge, respectively), a slower component is stored for millennia (with characteristic timescales of 2500 and 3300 ¹⁴C yrs for Cedar and Golden Ridge, respectively). Juxtaposition of young and old bank exposures throughout both tributary deposits attests to repeated cycles of localized deposition and partial evacuation that produce steady-state sediment "reservoirs" over centuries and longer times. Flux estimates derived from the inferred mean transit times indicate that most (>66%) of the sediment yield of the Cedar tributary is stored in the fan for some time but only a small part (3%) is stored in the Golden Ridge tributary terraces. Whereas fluvial deposits like that at Golden Ridge Creek provide a relatively weak buffer between their contributing areas and larger downstream channels, debris fans like that at Cedar Creek provide the major coupling between hillslopes in such tributary basins dominated by debris flows and mainstem channels dominated by fluvial processes. The accommodation space necessary for debris fans and other debris-flow deposits therefore imposes a strong control on mainstem valley and channel widths.
DE: 1115 Radioisotope geochronology
DE: 1810 Debris flow and landslides
DE: 1824 Geomorphology: general (1625)
DE: 1861 Sedimentation (4863)
DE: 1862 Sediment transport (4558)
SC: Hydrology [H]
MN: 2007 Fall Meeting