The Willamette River valley lowlands of Oregon are characterized by mineral soils, relatively flat topography, and distinct wet winter and dry summer seasons. Prior to European settlement, seasonal wetland prairie was a common sight across the older terraces of the Willamette River floodplain. Today, less than one percent of this unique ecosystem remains. Many former wetlands now exist as relict, semi-isolated, seasonally wet areas on farm land. Despite soil disturbance and reduced species diversity, these wetlands may retain many of the hydrologic functions found in their natural wetland counterparts. For both natural and farmed wetlands, we currently lack important information on water retention, groundwater interactions, and connectivity with other wetlands and surface water through the subsurface. Such attributes are often overlooked in wetland functional assessments based only on the surface water area and connectivity.

Developing a scientifically based, holistic approach to managing relict wetlands in catchments with a mosaic of land use types requires a better understanding of the mechanisms affecting surface and subsurface hydrologic storage across scales, and across distinct land cover and management types. In this study, we will examine the hydrologic role of remnant natural and farmed wetlands at two different scales. Our goal is to better understand and quantify how hydrologic storage and flow paths are influenced by wetland modification associated with land use change.

Study Plan

1) At the micro-site scale in a natural wetland, test which soil and biotic factors are most strongly associated with mechanisms controlling the inflow, outflow, and storage of water across the wet winter and dry summer seasons.

2) Use a sensitivity analysis to vary parameters related to agricultural land management to see how variables like soil mixing, compaction, tiling, and drainage affect the mechanisms by which the study wetland(s) delay and store runoff.

3) At the entire wetland scale, use tracers and hydrometric measurements to compare the residence time and flow paths of water in wetlands of the same soil and hydrogeomorphic type, but differing land management practices (natural, restored, and farmed).

This research will further our understanding of the hydrologic storage function of perched mineral wetlands under natural and farmed conditions, and how they interact with groundwater, streams, and other wetlands. At the wetland scale, the retention time and flow paths of water have implications for nutrient and pollutant transport and the support of wetland biological communities. At the catchment scale, connectivity between wetlands and other surface and ground water is also important in managing pollutant transport and sustaining the natural stream flow regime.