2008 Joint Meeting of The Geological Society of America, Soil Science Society of America, American Society of Agronomy, Crop Science Society of America, Gulf Coast Association of Geological Societies with the Gulf Coast Section of SEPM

Paper No. 18
Presentation Time: 8:00 AM-6:00 PM

Coupled Hydrologic and Landscape Recovery after Catastrophic Volcanic Disturbance

CUMMINGS, Michael L., Department of Geology, Portland State University, P.O. Box 751, Portland, OR 97207, CummingsM@pdx.edu

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.