HR: 0800h
AN: H51E-0808    [Abstracts]
TI: Comparison of Two Landslides and Related Outburst Flood Deposits and Their Effects on River Evolution, Owyhee River, Oregon
AU: * Othus, S
EM: othuss@cwu.edu
AF: Central Washington University, 400 E. University Way Central Washington University, Ellensburg, WA 98926, United States
AU: Ely, L
EM: ely@Geology.cwu.edu
AF: Central Washington University, 400 E. University Way Central Washington University, Ellensburg, WA 98926, United States
AU: House, K
EM: khouse@unr.edu
AF: Nevada Bureau of Mines and Geology University of Nevada, Nevada Bureau of Mines and Geology University of Nevada, Reno, NV 89557,
AU: Safran, L
EM: safran@lclark.edu
AF: Lewis and Clark College, 0615 S.W. Palatine Hill Rd, Portland, OR 97219, United States
AU: O'Connor, J E
EM: oconnor@usgs.gov
AF: US Geological Survey, 10615 SE Cherry Blossom Dr, Portland, OR 97216, United States
AU: Fenton, C
EM: cassiefenton@yahoo.com
AF: GeoForschungsZentrum, Telegrafenberg Haus B, Potsdam, D-14473, Germany
AB: Abundant channel-encroaching landslides and lava flows on the Owyhee River in southeastern Oregon have the capacity to both inhibit incision by altering channel slope, width, and bed character, and burying valley-bottom bedrock under exogenous material, and promote incision by generating cataclysmic floods through natural dam failures. We hypothesize that these extrafluvial events play a significant role in creating and maintaining the geomorphic features of river canyons in uplifted volcanic terraces that comprise a significant part of the western U.S. Numerous landslides have entered the Owyhee River canyon north of Rome, Oregon. As the river flows through different lithologic units, the style of mass wasting changes from large slump events to large debris flows. These differences seem to be related to the composition and thickness of the underlying, exposed Tertiary sediments relative to the basalt cap. The largest exemplary mass wasting events of each morphologic category in two reaches were examined to compare the effects of these events on the river channel. Multiple slump events ranging in age from approximately 10⁴ to 10⁵ ka have impacted the river channel near Artillery Rapid (River km 33 from Rome, OR). At least one of these appears to have dammed the channel and failed catastrophically, creating a flood bar immediately downstream, with boulders up to 3 m in diameter that decrease in size with distance from the landslide dam. Greeley Debris Flow (River km 70), the largest debris flow in the Hole in the Ground reach, blocked the channel, creating extensive fill terraces behind the landslide dam and a large depositional flood bar downstream, with an age possibly as young as early Holocene. Downstream of both landslide reaches, outburst flood deposits of large boulders armoring the channel were measured and used to determine the velocity of the outburst flood that entrained the boulders. Based on this evidence, it is clear that landslides have affected the evolution of both the stream channel and hillslopes. Comparing the different landslide types shows differences regarding slip surface geology and mass wasting stage. Approximately 270ft of the sediment package underneath the basalt cap at Artillery Landslide is exposed and shows coarser grained units with a mixture of lithics and volcaniclastic sediments, indicating fluvial emplacement. This specific reach has undergone only one generation of slump events. In contrast, the Greeley Debris Flow is located in an area where approximately 900ft of underlying sediment has been exposed by many undifferentiated mass wasting events. The sediments that create Greeley Debris Flow are composed of bentonitic clay lacustrine deposits. The presence of a well exposed and thicker sequence of silicic bentonitic clays at Greeley Debris Flow suggests (1) these large debris flow events are more common in areas where basalt overlies silicic lacustrine deposits and (2) there has been a longer and greater frequency of mass wasting at the Hole in the Ground reach resulting in secondary debris flows created from initial slump events.
DE: 1625 Geomorphology and weathering (0790, 1824, 1825, 1826, 1886)
DE: 1824 Geomorphology: general (1625)
DE: 1825 Geomorphology: fluvial (1625)
DE: 1826 Geomorphology: hillslope (1625)
SC: Hydrology [H]
MN: 2007 Fall Meeting