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History of Oregon's Wetlands

Oregon has always been known for its abundance of natural resources. Native Americans managed these resources for thousands of years by using fire and other tools. Wetlands were a major source of food and fiber, and most summer camps and permanent villages were situated adjacent to wetlands. Beginning about 1825, a trickle of Euro-American traders turned into a flood of farmers, ranchers, loggers, and miners who managed the land with a heavier hand. They and their descendants had to make a living, some of which caused adverse impacts to wetlands. However, 150 years of cumulative impacts have taken their toll on wetlands, and today we struggle to preserve remnants of many kinds of these habitats that have become rare.

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.

Historical Loss of Wetlands

Estimates of wetland loss in Oregon since 1850 vary by region and data source. Statewide, 38 percent of wetlands are thought to have been converted to other uses (Dahl 1990). Losses in various regions of the state vary from 57 percent in the Willamette Valley to 91 percent in the Klamath Basin, while losses for individual coastal estuaries range from 2 to 94 percent (Good 2000; Morlan 2000). Losses for particular rare wetland types such as wet prairie or peatland in the Willamette Valley range from 99.5 to 98 percent, respectively, and 88 percent of tidal spruce swamp has disappeared from the coast and lower Columbia River (ORNHIC).

Mudflat at Netarts Bay

Mudflat at Netarts Bay
(Heather Stout)

Existing regulatory programs have slowed but not stopped the loss of freshwater wetlands in Oregon. Estuarine wetlands are almost all protected by current land use laws, and impacts have been greatly reduced from what they were 30 years ago. However, rising sea levels associated with climate change may already be causing irreversible changes to these wetlands. Overall, wetland restoration programs are helping to reduce losses, and in some areas are making net gains in wetland acreage, but much work remains to be done to protect our wetlands.

For documentation of wetland trends in Oregon and nationally, the information is accessible from the Monitoring Trends page.

Sources for Spatial Information

Wetland and Hydric Soils of Oregon: Compiled by ORBIC and The Wetlands Conservancy, 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)