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RIPARIAN AREAS OF AN AGRICULTURAL LANDSCAPE IN WESTERN OREGON
Citation:
WIGINGTON JR, P. J. RIPARIAN AREAS OF AN AGRICULTURAL LANDSCAPE IN WESTERN OREGON. Presented at 2001 Annual Meeting of American Water Resources Association, Albuquerque, NM, November 12-15, 2001.
Description:
The Willamette Valley is a productive, diversified agricultural area in western Oregon. Pastureland and grass seed fields, mostly located on poorly drained soils, account for 60% of the agricultural land in the valley. The size and character of Willamette Valley streams and associated riparian areas range from intermittent streams originating in the valley floor, to perennial streams originating in the surrounding Cascade or Coast Range Mountains, to the mainstem of the Willamette River. Riparian area ecological processes and functions vary among stream types and landscape settings within the valley. This presentation provides an overview of a series of riparian studies conducted in the Willamette Valley during the last six years.
Intermittent streams with watersheds in poorly drained areas of the valley floor typically have streamflow from November through May. A winter aerial photography inventory of the extent of wet stream channels and swales revealed that wet season stream networks are often > 20% of watershed areas. Riparian forests generally do not occur along these intermittent streams. A field study of two sites showed that riparian areas with uncultivated grass or forb vegetation along intermittent streams can dramatically reduce nitrate concentrations of water draining from
grass seed agricultural fields but only a small portion of total streamflow passes through these riparian zones.
Perennial streams with watersheds draining the valley floor and foothills have riparian vegetation ranging from forests to agricultural fields. For 23 watersheds in the Willamette Valley, the amount of woody and uncultivated riparian vegetation along stream networks was correlated strongly to a fish index of biotic integrity (IBI) but did not have a strong correlation with winter (wet season) nitrate concentrations.
Remnant riparian forests occur along the lower reaches of the perennial streams draining from the Cascade and Coast Range Mountains and along the mainstem of the Willamette River. Examination of ~ 50 year historic aerial photographic record of the mainstem of the Willamette River, provided strong evidence that high flow hydrologic events and the associated movement of river channels are key processes in the establishment of floodplains and associated riparian forests. Field studies in the mainstem of the Willamette River and in the Calapooia River showed that water quality changes along hyporheic flow paths have important effects on river water quality and aquatic habitat.