Graduate Thesis Or Dissertation

Hydric Soils in Eastern Oregon Temporary Pools

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  • Two types of temporary poois, known as playas, occur in semiarid southeast Oregon. Vegetated upland playas are found in the High Lava Plain province; barren alkaline-saline playas are found in the Basin and Range province. Both types become ponded in most years, but they often lack redoximorphic features indicative of hydric soils. We studied the morphology, hydrology, temperature, chemistry, redox potentials, and vegetation in one pool of each type to elucidate processes of soil genesis, to determine if the soils are hydric soils, and to evaluate relevant field indicators of hydric soils. We collected data for 3 years from piezometers, Pt electrodes, and thermocouples installed at several depths at monitoring sites within and around the pools. Chemical and physical parameters of soil and water samples were analyzed. Plant species were identified and their cover estimated. The upland playa was ponded annually but never exhibited subsurface saturation. Its soils were hydric, meeting the hydric criteria for hydrology, redox potential, and temperature. Redox potentials indicated an anaerobic soil environment, but they were never low enough to significantly reduce iron. Depleted matrices and soft Fe-Mn concentrations were present; soft concentrations were too small and inconspicuous to meet the criteria of any current hydric indicators. We propose new indicators based on observed morphological features. Vegetation, soils, and hydrology at this site indicate great similarity with vernal pools in northern California. The pool meets all criteria for classification as a jurisdictional wetland. Effects of current management on the ecology of these pools is not known. The alkaline-saline playa was ponded annually and also saturated at depth. Its soils were hydric, but low soil organic carbon content, low soil Fe-Mn content, high pH, and redox potentials above the thresholds for Fe and Mn reduction suppress formation of hydric soil morphological indicators. Fe-Mn nodules, found throughout the profile, are probably not formed by redox processes. We propose that the limit of greasewood encroachment onto the playa be used as a field indicator of the boundary between hydric and non-hydric soils.
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