FENS, A PUMICE AQUIFER, AND HYDROLOGIC RESPONSE TO HOLOCENE VOLCANISM, SOUTH-CENTRAL OREGON

There are fens and seasonal wetlands at the headwaters of the Klamath and Deschutes river basins in the rain shadow of the Cascade Range in south-central Oregon. The wetlands are on an uplifted fault block that was blanketed by 2.5 to 3.5 m of pumice during the Holocene eruption of Mount Mazama. The pumice forms a weakly confined perched aquifer which is associated with slope fens marked by diffuse groundwater flow through up to 1.4 m of peat. The fens are areas of high biodiversity for the region, but the hydrological system that supports these wetlands and the landscape response to the volcanic inundation is not well understood.

Round Meadow, a 1.2 km² basin at 1715 m elevation is the highest wetland in the region and has been studied over a period of 3 years to help describe the hydrology of the larger system. An array of 14 piezometers was installed to monitor the water table and hydrochemistry in the basin. A water budget will be calculated, and the piezometer array was surveyed to create a map of the water table. O and H isotope ratios were tested for springs, streams, and groundwater to predict storage time and fate for water in the meadow.

The pre-eruptive surface is marked by a sharp contact with well sorted phenocryst and lithic rich sand, which underlies the lower pumice unit, 1.5 to 2 m of moderately sorted coarse pumice lapilli (0.2 to 0.7 cm). The upper pumice unit, 1 m of poorly sorted lapilli to blocks (0.2 to 6 cm), sparse phenocrysts, and lithics is present on the slopes surrounding Round Meadow, but not within the meadow. In the basin the upper pumice has been replaced by 1 – 1.2 m of alluvium which grades from vitric silt to sub-angular pumice sand and directly overlies the lower pumice. This contact is an erosional surface that cuts more deeply into the pumice moving down from the slopes to the basin. Lower pumice is the aquifer material and hydraulic head is higher in piezometers screened in the pumice than for those in the overlying alluvium. Electrical conductivity in groundwater increases through the dry season and typically ranges between 20 and 50 μS/cm, but rare conductivities above 200 μS/cm were found. 3 springs in Round Meadow are at the contact with the bedrock that forms the northern margin of the basin. These springs are all isotopically similar, but distinct from groundwater in the pumice aquifer and from surface flow leaving the meadow.