Recent wetlands trends (1981/82–1994) in the Willamette Valley, Oregon, USA

Abstract

A two-stage, stratified, systematic sample design was implemented in the Willamette Valley, Oregon, USA, to quantify wetland and land-use changes from the 1980s to the 1990s. The Stage 1 sample (n=711) was drawn from public land survey sections and was stratified by land use and runoff potential. The Stage II sample (n=114) re-sampled the Stage I sample stratified by the amount of hydric soils identified in the Stage 1 sample. Wetland and upland classes were delineated on large-scale aerial photographs, digitized into ARC/Info coverages, and compared to quantify land-cover changes. Total loss of wetlands to uplands during the study period was about 3,800 ha, representing a 2.1 percent wetland loss from the 1980s. The net loss after adjusting for wetland gains was about 2,750 ha. During the study period, 70 percent of the wetland loss was associated with agriculture, six percent was lost to urbanization, and 24 percent was lost to other changes. The loss of wetlands to agriculture and the conversion of wetland types was consistent with a pronounced climatic component related to below-normal precipitation from 1985 to 1994, although continued installation of tile drains and expansion of irrigated agriculture also may have contributed to the changes. The loss of wetlands to agriculture raised questions regarding the effectiveness of current agricultural wetland policy, which appears ill-prepared to protect small wetlands or to deal with loss of wetlands from intensified use of existing farmland. This study identified a larger number and area of wetlands compared with national wetland surveys because of the larger scale data used in this study, the nature of the strata used in the statistical design, and the inclusion of palustrine farmed wetlands in the landuse classification.