- 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
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.