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FLUVIAL DISTURBANCE AND WETLAND VEGETATION DEVELOPMENT, UPPER MAIN STEM, WILLAMETTE RIVER, OREGON, USA
Citation:
CLINE, S. P. FLUVIAL DISTURBANCE AND WETLAND VEGETATION DEVELOPMENT, UPPER MAIN STEM, WILLAMETTE RIVER, OREGON, USA. Presented at Annual International Meeting of the Society of Wetland Scientists, Sacramento, CA, June 10 - 15, 2007.
Description:
Hydrogeomorphic processes drive vegetation establishment, and promote development of diverse wetland and riparian types associated with lotic ecosystems. The main objective of this study was to estimate the rate and pattern of vegetation development on bars tracked since 1936, and secondarily, to relate vegetation to the historic fluvial disturbance regime in the upper main stem of the Willamette River. The time period brackets the transition from relatively high to low hydrogeomorphic potential due to dams and revetments. There was a significant positive, linear correlation between independent estimates of the age of bars from aerial photos and the age of colonizing vegetation from increment cores. The composition of vegetation differed with bar age, becoming dominated by upland species over time. Vegetation communities aligned primarily by bar elevation above summer low flow, and secondarily, by the level of exposure to erosion. Notably, there was a positive relationship between annual precipitation, peak and bankfull discharges, and the frequency of fluvial disturbances that rejuvenate vegetation. These relationships suggest that plans to increase the Willamette's hydrogeomorphic potential via restoration of pre-dam hydrology are possible, and that the magnitude and frequency of peak and bankfull discharges during cool, wet years are initial targets to consider for the maintenance of diversity of types and age classes of native plant communities.