301-18 Coupled Hydrologic and Landscape Recovery after Catastrophic Volcanic Disturbance

Poster Number 63

See more from this Division: General Discipline Sessions
See more from this Session: Hydrogeology (Posters)

Wednesday, 8 October 2008
George R. Brown Convention Center, Exhibit Hall E

Michael Cummings, Department of Geology, Portland State University, Portland, OR
Abstract:
Coupled hydrologic and landscape recovery after the catastrophic eruption of Mount Mazama in the Cascade Volcanic Arc of Oregon at approximately 7627±150 cal. yrs. B.P. began shortly after pyroclastic flows of the ring-vent phase inundated the low-relief Williamson River basin and filled and overtopped the 30 to 60 m deep, narrow (220-275 m), bedrock-lined outlet canyon. The debris dam, composed primarily of unsorted pumice and ash, is estimated at 52 m to 58 m thick, 2.4 km long, and containing 4.4 x 107 m3 of debris. The impoundment attained maximum area of 590 km2 and 30 m depth before the dam catastrophically failed. Peak discharge at the breach is estimated at 1.3 x 104 m3s-1, draining the lake in 2.5 days.

Contemporaneous erosion of thick pyroclastic deposits in the Cascade Range contributed glass- and crystal-rich (phenocrysts) silty sand to thin (1-2 m) alluvial fans expanding over pyroclastic-flow deposits along the range from (western edge of basin). Streams extending beyond these fans cut into pyroclastic-flow deposits and constructed distal alluvial fans on the already drained lake bed. Subsequent incision through range front fans and pyroclastic-flow deposits redirected surface water to the ground water system leading to abandonment of canyons in the range front and the early-formed channels in the basin.

In the pumice-dominated eastern basin, bedrock distribution of the pre-eruption landscape influenced where snow-melt driven stream flow eroded channels and deposited alluvial fans. After the spring freshet, the water table retreats into the pumice and pre-eruption alluvium where slow ground water flow continues.

Post-eruption faulting impounded the Williamson River on the down-thrown block to form Klamath Marsh, a major wetland complex. Ground water migrating from the Cascade Range through pumice deposits discharges to the marsh and controls water levels during the growing season.

See more from this Division: General Discipline Sessions
See more from this Session: Hydrogeology (Posters)