Geology,   Hydrology   &   Landscapes

Wetland prairies are typically located on poorly drained, very low gradient lands found in the valley bottom. They are generally found on expanses of heavy clay soils, but can also occur on well-drained soils with shallow bedrock impeding subsurface drainage, or along swales or drainages. Although limited published research exists on the subject, soil scientists conducting mapping for the Lane County soil survey noted considerable complexities in the geomorphic surfaces found in soils associated with wetland prairies, particularly in the southern end of the Willamette Valley.

The variation in topography, up to one-half meter, and associated variation in surface hydrology, tends to promote much greater degree of plant species diversity than in areas with more uniform topography. Although these topographic traits were not specifically mentioned in the GLO survey notes, the Calapooyia geomorphic surface is visually evident on many 1936 and 1940 aerial photos taken in the southern Willamette Valley.

Much of this geomorphic surface has been modified over the past several decades as most agricultural fields in the area have been mechanically smoothed in an effort to improve conditions for grass seed production.

Topographic variation also exists on a much finer scale, as many wetland prairies also contain a complex vertical structure in the form of raised pedestals. It has been widely observed that over time, a pattern of raised pedestals 3 to 20 centimeters in height form above a lower level of soil within many wetland prairies. These raised areas range from 15 to 400 square centimeters in area and tend to remain above water between winter storms, resulting in several types of microhabitats in a relatively small area.

Grasses and forbs tend to thrive on the higher portions of the pedestals, while more water tolerant rushes, sedges, and annual forbs are often found in the low spaces between pedestals, which are inundated for much of the wet season (Wilson 1998).  The origin of this pedestal-interspace microtopography is unknown, but observations suggest that the formation of mature clumps of bunch grasses such as Deschampsia cespitosa (tufted hairgrass) and/or large ant mounds may play a role as they interact with site hydrology.

Wetland restoration practitioners have observed that pedestals will often begin to form within a few years following the re-establishment of wetland prairie vegetation in previously cultivated and flattened grass seed fields in the West Eugene area.