Graduate Thesis Or Dissertation
 

An edaphic study of the Mt. Pisgah Arboretum water garden, Coast Fork of the Willamette River

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  • Wetlands are widely identified as providing important and fundamental processes valuable for maintaining ecosystem health and diversity. Located in the southern Willamette Valley, the Mt. Pisgah Arboretum contains some valuable remaining wetland habitat along the Coast Fork of the Willamette River. One goal of the Mt. Pisgah Arboretum is "to promote conservation, research, and awareness of ecology". To reach this goal, the Arboretum has identified the importance of maintenance and enhancement activities for onsite native habitats, including riparian and wetland habitats. Before restoration or enhancement activities can begin, it is essential to develop an understanding of current environmental conditions. The purpose of this research was to document both the characteristics and distribution of hydric soils and the hydrology, and to provide insight into the patterns and processes associated with a floodplain wetland. In this study, transect sampling of edaphic features was used to identify the distribution of hydric soils and the hydrologic nature of the Mt. Pisgah Arboretum Water Garden. Soil morphological data for particle size, matrix colors and redox features were evaluated and compared with observations of ground water hydrology, river hydrology and precipitation. Five stratigraphic units were identified underlying the Water Garden. A basalt Bedrock unit underlies the uplands associated with Mt. Pisgah and extends at least part way beneath the floodplain. The Clay unit was formed above the Bedrock unit, with some degree of encroachment onto the floodplain. Below 153 m are floodplain sediments, cobbles at depth, then a sand layer and silty clay loam at the surface. The Cobble unit overlaps the Bedrock unit at its base and is most likely Pleistocene age alluvium. The Sand unit is of Holocene age and is found only in the abandoned thalweg, tapering off laterally in both directions across the ancient channels. Draped above this all and slightly overlapping the upland Bedrock and Clay units, is the SiCL unit. The SiCL unit represents Holocene age alluvium, fine material deposited by slow moving water and overbank deposition. The Water Garden soils reflect this mosaic of parent materials on a complex slope. Water Garden soils sometimes met saturation requirements for hydric soils, but they did not always meet hydric soil indicator requirements. The hydrological data suggest that the soils in depressional areas of the Water Garden occupy a zone where water is exchanged between saturated sediments surrounding the channel of the Coast Fork and the channel itself. The hydrology of depressional areas with both ponded surface water and near surface saturation was principally the result of hyporheic upwelling. The soils in these depressional areas tended to form redox concentrations that met hydric soil indicator criteria. Hillslope soils in concave footslope positions exhibited hydrology indicating two separate zones of saturation, one near surface, the other at depth, related to infiltration and accumulation of precipitation. Few redoximorphic features were observed in hillslope soils, and the one hydric soil indicator that was used at these locations did not require redox. Accurate and detailed delineation of hydric soils on this landscape and clear determination of dominant sources of saturation provided an improved understanding of the complex nature of the Water Garden wetland. Results of this study show that hydric soils occupy both depressional and hillslope positions within the Water Garden. Delineation of a soil as hydric or non hydric was facilitated by the use of hydric soil indicator criteria, morphology and hydrology. This analysis provides the managers of the Mt. Pisgah Arboretum with an accurate representation of where hydric soils currently exist and the respective sources of saturation. With this information, managers are better equipped to develop restoration and enhancement options that better reflect the current environmental conditions in the Water Garden.
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