This list of ecoregions in Oregon provides an overview of ecoregions in the U.S. state of Oregon designated by the U.S. Environmental Protection Agency (EPA) and the Commission for Environmental Cooperation (CEC). The Commission's 1997 report, Ecological Regions of North America, provides a spatial framework that may be used by government agencies, non-governmental organizations, and academic researchers as a basis for risk analysis, resource management, and environmental study of the continent's ecosystems. Ecoregions may be identified by similarities in geology, physiography, vegetation, climate, soils, land use, wildlife distributions, and hydrology.
The classification system has four levels. Levels I, III, and IV are shown on this list. Level I divides North America into 15 ecoregions; of these, 3 are present in Oregon. Level III subdivides the continent into 182 ecoregions; of these, 9 lay partly within Oregon's borders. Level IV is a further subdivision of Level III ecoregions. There are 65 Level IV ecoregions in Oregon, many of which continue into adjacent areas in the neighboring states of Washington, Idaho, Nevada, and California. The task of defining and mapping these ecoregions was carried out by the Oregon Ecoregion Project, a collaborative effort involving the EPA, the United States Geological Survey (USGS), the United States Forest Service (USFS), and other state and federal agencies. The new classification system they developed may differ from previous frameworks developed separately by the agencies.
Oregon is ecologically diverse. The west side of the state has a marine-influenced climate and receives plentiful precipitation three seasons of the year. In contrast, Eastern Oregon lies in the rain shadow of the Cascades and is much drier. The climatic gradient is evident in the state's landscapes: forested mountains, glaciated peaks, shrub- and grass-covered plains, agricultural valleys, beaches, desert playas, and wetlands.