HR: 0800h
AN: H51E-0807    [Abstracts]
TI: Post-Mazama Break-out Flood on the Sycan River, southern Oregon
AU: * Lind, P
EM: plind@uoregon.edu
AF: University of Oregon, Geography Dept 1251 University of Oregon, Eugene, OR 97403,
AU: O'Connor, J E
EM: oconnor@usgs.gov
AF: US Geological Survey, 2130 SW 5th, Portland, OR 97201,
AU: McDowell, P
EM: pmcd@uoregon.edu
AF: University of Oregon, Geography Dept 1251 University of Oregon, Eugene, OR 97403,
AB: The Sycan River is the largest tributary to the Sprague River watershed in the semi-arid Upper Klamath Basin of south-central Oregon. Only 56 kilometers (35 miles) from Crater Lake, Mt Mazama's&premnant volcanic crater, the headwaters of the Sycan watershed has an abundance of aeolian and fluvially distributed Mazama pumice. The upper Sycan River flows through a wide basin 17 km long (Sycan Marsh), then southward through 40 kms of narrow basalt canyon. The lower16 river kilometers occupy a low gradient unconfined southward-widening valley at the Sprague River confluence. Field investigation of cut banks and auger holes in the lower valley reveal an extensive 1 to 2-m thick lobe-shaped deposit dominated by Mazama pumice, and fining upward from granules to medium sand. This deposit is interpreted as resulting from a single or set of break-out flood events relatively soon after the pyroclastic eruptions of Mt Mazama 7,700 years ago. The flood event buried a fine-grained floodplain with a thick layer of pumiceous sands across the river's unconfined lower valley section. In the upstream canyon section, boulder bars and boulder terraces as well as terraces of Mazama tephra indicate that the event was a relatively large system-wide flood. Distribution and stratigraphy of the resultant flood terraces of the lower Sycan River suggest that the event was a break-out flood caused by blockage in the confined canyon section. The location and mechanism of blockage are not definitely known; it may be either landslide(s) from the canyon walls, or accumulation(s) of Mazama tephra and other sediments in the canyon. Submitted radiocarbon samples from or near the base of the flood deposits will date the break-out flood event. Additional analysis and field investigation is underway to determine the mechanism and location of the canyon blockage, the hydraulic dynamics of the flood and the extent of the flood deposit lobe.
DE: 1820 Floodplain dynamics
DE: 1821 Floods
DE: 1824 Geomorphology: general (1625)
DE: 1856 River channels (0483, 0744)
SC: Hydrology [H]
MN: 2007 Fall Meeting