TITLE:
Hydrogeology of Pumice-Hosted Fens in the Winema-Fremont National Forest, Oregon, USA
AUTHORS:
Michael L. Cummings, Jonathan M. Weatherford, Leslie A. Mowbray
KEYWORDS:
Groundwater Dependent Ecosystem, Fen, Pumice Aquifer, Mount Mazama, Oregon
JOURNAL NAME:
Journal of Water Resource and Protection,
Vol.6 No.19,
December
30,
2014
ABSTRACT: Subaerial
fallout from the Holocene eruption of Mount Mazama in the Oregon Cascade Range
was deposited upon relatively low permeability volcanic and volcaniclastic
bedrock and regolith. In the Walker Rim study area, erosion by ephemeral
streams shortly after the eruption disrupted the lateral continuity of the 270
to 300 cm-thick pumice deposit. Co-evolution of the surface- and ground-water
systems in a low-relief, low-slope landscape allowed diffuse groundwater
discharge from the banks of the evolving stream system. Accumulation of organic
material from groundwater dependent ecosystems at these sites of discharge
allowed peat deposits to form on gently sloping erosion surfaces cut into the
pumice deposit. Following early stream incision, fine-grained, silt-rich
deposits accumulated in valleys and contributed permeability barriers to the
lateral migration of water in the pumice aquifer. Fens discharge from the
pumice aquifer through gently sloping surfaces patterned after the slope of the
erosion surface cut into the pumice deposit and overlain by approximately 1 m
of peat on the sloping surface and alluvium or iron-cemented pumice overlain by
alluvium at the toe of the slope. The predominant source of groundwater is snowmelt
which infiltrates the pumice deposit during the freshet. However, shallow
groundwater flow also takes place along permeable pathways in bedrock units.
Locally, low volume discharge takes place along faults. The snowmelt-dependent
hydrologic system that supports the fens of the Walker Rim study area occurs at
elevations primarily above 1585 m.