Graduate Thesis Or Dissertation
 

Investigations of soil morphology, hydrology, reduction-oxidation potentials, and stratigraphy on a selected hillslope in western Oregon

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  • Hydric soils are defined through the hydric soil definition, and a means to identify hydric soils has been established through hydric soil criteria and Field Indicators of Hydric Soils in the U.S. However, the field indicators are a recent development that requires more research and testing to increase our knowledge of correlations between soil morphology, hydrology, and soil processes and properties for all types of landscapes and geomorphic processes. The main objective of this study was to evaluate the Field Indicators capability to identify those soils that met the hydric soil definition on a selected hillslope. Further research and investigation was done to determine whether the study area would meet federal specifications for jurisdictional wetlands and assess stratigraphic units and geomorphic processes that contributed to the present hydrological conditions. Morphological observations; soil physical, chemical, and mineralogical analysis; collection and analysis of piezometric, precipitation, soil temperature, and reduction-oxidation potential data; vegetation characterization; and geomorphological and stratigraphical investigation were used to characterize the soils and study transect (Sites 1 through 4) on the backslope-footslope of Witham Hill in Corvallis, Oregon. Hydrologic, redox potential, and soil temperature data provided documentation that the soils of Sites 2, 3, and 4 meet the conditions in the hydric soil definition. Seasonal perched water tables ranging from 9.7-12.6 cm occur over a discontinuity consisting of slowly permeable clays on the upper footslope; and temporary episaturation that gives way to endosaturation with an average water table at 5.5 cm occurs on the lower footslope. Redox data that indicated continuous iron reduction for 21 to 30 weeks correlated with the morphological properties for each of the three sites. However, morphological characteristics of the soils on the upper footslope did not correspond with the Field Indicators of Hydric Soils in the U.S., Version 3.2, 1996. Two major factors that prevented positive outcomes for any indicators were layer thickness requirements and the inability to round a color that fell between color chips. A third factor was the requirement that at least 60% of a designated layer have a depleted matrix. The first two issues were addressed by the Field Indicator Committee of the National Technical Committee for Hydric Soils in the Field Indicators of Hydric Soils in the United States, Version 4 issued in March 1998. Further investigation into study area characteristics found that: (1) physical and mineralogical analysis supports the presence of four stratigraphic units on the upper footslope; (2) mineralogy indicates that the two clay units and the Spencer Formation that underlies the area may share a common origin and that the clay units could be colluvial material from the eroded Spencer Formation of once-higher surfaces; and (3) soils at Sites 2, 3, and 4 meet the three criteria for identification of jurisdictional wetlands.
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