Poster Number 63
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.