HR: 0830h
AN: H31C-0477    [PDF]
TI: Effects of Alluvial and Debris Flow Fans on Channel Morphology in Idaho, Washington, and Oregon
AU: * Bigelow, P E
EM: paulbigelow@siskiyou.net
AF: Earth Systems Institute, 310 N. Mt Shasta Blvd, #6, Mt Shasta, CA 96067 United States
AU: Benda, L
AF: Earth Systems Institute, 310 N. Mt Shasta Blvd, #6, Mt Shasta, CA 96067 United States
AU: Miller, D
AF: Earth Systems Institute, 3040 NW 57th St., Seattle, WA 98107 United States
AU: Andras, K
AF: Earth Systems Institute, 310 N. Mt Shasta Blvd, #6, Mt Shasta, CA 96067 United States
AB: Formation of debris flow and alluvial fans at tributary confluences from episodic erosion associated with large storms and fires ("extreme events") are often viewed negatively over short time spans (years). However, when viewed over long periods of time (decades to centuries), fans that form at tributary junctions are often sources of morphological diversity in streams and rivers. To evaluate effects of tributary fans on the morphology of mainstem channels, we surveyed a total of 44 km of streams in the Sawtooth Mountains of Idaho (27 km), Olympic Mountains of Washington (10 km), and Central Coast Range of Oregon (7 km). Rejuvenated alluvial fans resulting from post-fire gully erosion in the Sawtooth Mountains created gradient nick points in 4th to 6th order mainstem channels (30 to 350 km$^{2}$ drainage area) that increased sediment storage upstream resulting in decreased channel gradients, widened flood plains, side channel construction, and the beginning of terrace formation. Downstream effects included increased channel gradients, often creating rapids. In 3rd and 4th order mainstem channels ($<$ 10 km$^{2}$ drainage area) in the Olympic Mountains, there was statistically significant association between low-order confluences containing debris flow deposits and gravel abundance, wide channels, and numbers of logs and large pools. Moreover, heterogeneity of mainstem channel morphology increased in proximity to low-order confluences prone to debris flows in the Olympic study sites. In 3rd and 4th order channels in the Oregon Coast Range, density of large wood and boulders in mainstem channels ($<$ 30 km$^{2}$ drainage area) increased with proximity to all debris flow fans at low-order confluences regardless of fan age, while channel gradients and sediment depth in mainstem channels increased with proximity to recent ($<$ 60 yrs old) debris fans. Consequently, alluvial and debris flow fans can be significant agents of heterogeneity in riverine habitats, similar to other sources of major gradient nick points on mainstem channels (e.g. bedrock, rock falls, canyon constrictions, channel bends, etc.). However, not all channels are prone to tributary fan effects. Steep and confined mountain channels with high stream power may quickly transport deposits from debris flow and alluvial fans, leaving no morphological effects. Overall, these field studies provide a possible physical basis for recent observations of increased habitat use near tributary junctions (e.g. salmon spawning density, aquatic invertebrate density) and underpin emerging stream network theory on the organization of disturbance in creating and maintaining a variety of habitat in aquatic and riparian ecosystems.
UR: http://leebenda.siskiyou.net
DE: 1815 Erosion and sedimentation
DE: 1821 Floods
DE: 1824 Geomorphology (1625)
DE: 1869 Stochastic processes
SC: Hydrology [H]
MN: 2003 Fall Meeting