Stratigraphy and hydrology of the Jackson-Frazier wetland, Oregon

The relationship between wetland soils and hydrology can be better understo od by linking soil geomorphological features to the measurement of groundwa ter depths in the soil. Soil stratigraphic analysis and long-term measureme nts of soil water levels in piezometers were used at the Jackson-Frazier we tland in western Oregon to investigate the interaction between local geomor phological history and the hydrology of the wetland. Morphological descript ions confirm the presence of a recent smectitic alluvial deposit (80-180 cm ) overlying Malpass clay (approximate to 35 cm thick), which overlies Irish Bend Silt. X-ray diffraction and isotope analysis support the conclusion o f the presence of the Holocene alluvium and Irish Bend Silt, but are inconc lusive regarding the Malpass clay. Piezometer data from 1992 to 1996 show t hat the smectitic alluvium controls saturation and drying of the wetland su rface, and that a separate water table is present below the Malpass clay in the Irish Bend silt, The recent alluvium and Malpass clay act as an aquita rd that restricts the vertical infiltration of surface water and helps rest rict the groundwater table in the Irish Bend silt deposit. These stratigrap hic relationships and associated hydrologic responses provide a means to id entify wetlands and predict hydrologic conditions on similar wetland landsc apes.