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Historical Extent of Wetlands

For Oregon, the public land surveys of the General Land Office (GLO) are usually the best source of information on the historical landscape, but their information on wetlands is inconsistent because of differences in surveyors, the time of year in which surveys were conducted, and the frequent lack of information for the interiors of sections (usually one square mile each) that the surveyors were not required to record in detail. The early navigation charts of the U.S. Coast Survey, later known as the U.S. Coast and Geodetic Survey, are excellent sources for wetland extent and structure, but these are only available for nearshore line-of-sight features along the coast and the Columbia and Willamette rivers.

Woahink Bog

Woahink Bog
(Esther Lev, The Wetlands Conservancy)

The limitations of the GLO and Coast Survey maps for wetland data leave, by default, the soil surveys of the U.S. Department of Agriculture's Natural Resources Conservation Service (NRCS) and its predecessor agencies (Soil Conservation Service, Bureau of Chemistry and Soils, Bureau of Soils). Mapping of wetlands in early engineering studies are also very useful, but these are few and usually cover only limited areas.

Mapping of wetland soils is the most useful source of information on the historical extent of wetlands because these soils retain distinctive characteristics even after they have been drained and converted to other uses. Current NRCS mapping of wetland (hydric) soils shows those that are currently drained, diked (classified as "protected"), and farmed, and to a lesser extent those that have been filled. Soils were mapped using a combination of air photo interpretation, landforms, vegetation, and ontheground soil profiles. The map unit polygons, though by necessity somewhat generalized and coarse, remain the single best estimate of the extent of these soil types.

Sources for Spatial Information

Wetland and Hydric Soils of Oregon: Compiled by ORBIC and The Wetlands Conservancy and available from the Oregon Spatial Data Library, this data layer includes all currently available mapping by NRCS, supplemented with wetland soil and wetland habitat mapping by the USDA Forest Service and Weyerhaeuser Company. More detailed information about the NRCS map units in this layer are available from the NRCS Digital Soils Mapping for Oregon. Portions of Wheeler, Crook, and Malheur counties are not yet available. NRCS coverage generally excludes National Forests, but includes BLM lands.

Historical Wetlands: Wetlands and associated vegetation for portions of Oregon have been mapped based on interpretation of General Land Office (GLO) survey data, available on ORBIC's GLO web page. Where needed, mapping of historical wetlands was augmented with NRCS hydric soils data.

Authored by John A. Christy, Wetlands Ecologist, Oregon Biodiversity Information Center and The Wetlands Conservancy (2010)