HYROGEOLOGIC RESPONSE TO THE HOLOCENE ERUPTION OF MOUNT MAZAMA, CASCADE VOLCANIC ARC, OREGON

Hydrogeologic response in the rain shadow of the Cascade volcanic arc to the climactic eruption of Mount Mazama (7700 yrs ago) reflects pre-eruption geomorphology, distribution of pre-eruption stratigraphic units, and characteristics of pyroclastic-flow and –fall deposits. Immediately following the eruption a 590 km² lake formed behind a debris dam of pyroclastic-flow deposits. The lake persisted between 10 and 29 years before catastrophically draining. Along the east-facing range front, early-formed streams cut down through lower-permeability pyroclastic-flow deposits and eventually drained to underlying highly permeable pumice deposits. Where pyroclastic-fall deposits blanket the landscape 55 to 60 km NNE of Mount Mazama a discontinuous, unconfined, perched aquifer developed in 2 to 3 m thick pumice deposits. The aquifer developed concurrently as streams cut valleys into the pumice deposits. Discharge to streams from the evolving aquifer encouraged plant colonization and development of organic-rich sediment on sloping valley walls. Early down cutting was followed by deposition of crystal-rich sand grading upward to glassy silt with rounded pumice, crystals, and small lithic fragments. Alluvium directly overlies the pre-eruption surface, lower well-sorted pumice lapilli (3 to 7 mm) unit, the middle moderately well sorted pumice lapilli unit (5 mm to 20 mm), or upper, poorly sorted, lapilli to block unit (10 mm to 64 mm). Where slopes were low alluvial fans buried the upper pumice unit. Modern valleys include 1) seasonally dry valleys cut through the pumice that carry water only during the spring freshet, 2) valleys that carry water during the spring freshet and harbor groundwater in the lower pumice unit during the dry season, 3) broad pre-eruption valleys where alluvium commonly overlies saturated complete pumice sections, and 4) valleys that are perennially wet where the unconfined aquifer discharges as fens from the lower, middle, and upper pumice units. These fens support high diversity, ground-water dependent ecosystems. Recharge occurs during spring snow melt. Once in the unconfined aquifer groundwater may infiltrate to deeper levels, be consumed by evapotranspiration, migrate through the unconfined aquifer along seasonally connected flow paths, or return to the surface at fens and springs.