Wetlands are threatened by both direct and indirect impacts related to land use. Direct impacts include conversion of wetlands to other uses, primarily urban and agricultural. Indirect impacts originate from surrounding land uses, based on the fact that what occurs on surrounding uplands usually ends up in wetlands. Sediments, nutrients, deicer salts, petroleum products, and agricultural chemicals from uplands all move downslope and downstream, and some even move into groundwater.
The most common and pervasive direct threats to wetlands are channelization, sedimentation, filling, drainage, mining, logging, overgrazing or trampling by livestock, and conversion to agricultural and urban uses. Indirect threats include groundwater pumping, diversion of water for domestic or agricultural use, plant succession occurring in the absence of periodic fire or other natural disturbance, invasion of exotic species, changes in water regimes caused by upstream dams or other development, and pollution caused by direct discharge into wetlands or runoff from uplands. Changing climate poses many potential problems for wetlands, but specific effects remain unknown.
With new laws, the loss of wetland acreage in Oregon has slowed. However, despite the decrease, there is still some loss of both wetland acreage and function as a result of development and changes in land use. Indirect threats like invasive species and urban storm water and agricultural runoff impact the health of Oregon’s wetlands.
Alkaline wetlands are restricted to arid lands east of the Cascade Range. They include seasonally or intermittently flooded playas, marshes, and lakes, where alkaline soils and intense evaporation tend to concentrate salts in soils and water. They may support large populations of plants and animals found nowhere else in arid regions, and in Oregon they are particularly well known as breeding or foraging sites for vast quantities of migrating birds.
Aquatic beds are composed of mostly submerged vegetation, the upper portions of which may float at the surface. They occur in near shore areas along the coast, in estuaries, and in rivers, ponds, lakes, and sloughs on most topographic surfaces throughout the state. Plants may be rooted, suspended in the water column, or free-floating.
Fens Most wetlands in Oregon that have been called “bogs” are actually fens, because they are hydrated by mineral-rich surface or groundwater, lack a domed peat profile, and have a pH generally higher than 5.5. Many fens, however, contain localized hummocks or lawns of Sphagnum with a pH as low as 4, and these are classified as “poor fens.” Oregon’s fens occur in depressions and on slopes hydrated by springs and seeps, particularly in interdunal troughs, headwall basins, and floodplains. Fens may include patches of shrub swamp and forested wetland occurring on peat soils.
Forested wetlands are sometimes called swamps, and occur on seasonally or perennially wet flats, depressions, or stream terraces. Hydration may originate from precipitation, groundwater discharge, or inflowing streams. Forested wetlands sometimes occur within riparian zones but differ from riparian wetlands in their higher water tables and longer duration of surface water. Forested wetlands are typically flooded for several weeks during the growing season (seasonal flooding).
Freshwater marshes occur in depressions and around the edges of lakes, ponds, rivers, and streams where surface water is present for all or most of the growing season and the soil are perennially wet. They are characterized by emergent herbaceous vegetation, usually spikerush (Eleocharis spp.), sedges (Carex spp.), bulrushes (Schoenoplectus spp.), bur-reed (Sparganium spp.), cattails (Typha spp.), and various grasses. Broadleaved herbs and shrubs may also be present. They are particularly well known as breeding or foraging sites for birds. Freshwater marshes are found throughout the state, but many thousands of acres have been diked, drained, and farmed.
Freshwater tidal wetlands occur in the intertidal zone along coastal rivers, upstream from the estuarine system where salinity ranges from 0.5 ppt to 0 ppt. They extend along the edges of rivers and their tributaries to the head of tidal influence, which can be up to 20 miles inland along the shorter coastal river to 143 miles inland along the Columbia River. The tidal cycle is similar to that on the coast, but timing is delayed and the amplitude of tidal surge attenuates further upstream to about 3 feet at Portland and 1 foot near Bonneville Dam. Freshwater tidal wetlands can be herbaceous, shrubby, or forested, and like salt marshes are hydrated by meandering tidal creeks. Plant species diversity in these wetlands is extremely rich, and many of thousands of acres have been diked, drained, and farmed.
Salt marshes and brackish marshes occur in the intertidal zone of coastal estuaries. They are dominated by salt-tolerant herbaceous plants and are usually intermixed with intertidal mudflats that are rich with invertebrates and seaweeds. Farther inland in an estuary, salinity becomes diluted by fresh water from rivers and streams, and a gradual transition to freshwater plants and animals occurs, and woody vegetation becomes more evident. This transition zone is a moving target in upper estuaries, and is influenced by seasonal or episodic river flows and fluctuations in daily tidal cycles. With the anticipated rise in sea level associated with global climate change, many salt and brackish marshes may become permanently inundated, and their landward edges will advance into what are currently freshwater marshes, shrub swamps, and uplands. The estuarine-freshwater transition zone will also migrate upriver with rising sea level.
Serpentine fens are one of Oregon’s rarest wetland types. They form on slopes or in small basins where seepage and springs discharge in soils derived from peridotite and serpentinite bedrock. The soils have very high levels of base metals and very low levels of common nutrients needed by plants. These wetlands are restricted to a small area of Josephine County in southwestern Oregon and several other sites in the Klamath Mountains of northwestern California. Best known for their stands of cobra lily (Darlingtonia californica) that also occurs in coastal fens without serpentine, they are habitat for several species of plants found nowhere else on the planet. Primary threats to serpentine wetlands are groundwater pumping and diversion of spring water for domestic or agricultural use, and plant succession occurring in the absence of periodic fire.
Shrub swamps and shrub riparian occur along rivers and streams throughout the state and are often intermixed with upland portions of floodplains in a jumble of units that are difficult or impossible to map separately. Shrub swamps and shrub riparian are usually associated with seasonal flooding by streams and rivers along which they occur, but they may also be hydrated by perennial or seasonal seepage, tributary streams, or flooding caused by tidal cycles. Vegetation may be forested, scrub-shrub, or herbaceous, but is usually a mixture of alternating patches of all three different types. Historically, most riparian areas were grazed intensively by livestock, and severely degraded by soil compaction, denudation, down cutting of streams, and subsequent invasion by upland or exotic species. These impacts also degraded streams and caused problems for fish and other aquatic species. Over the last 20 years, extensive areas of riparian vegetation and hydrology have been restored by improved management practices.
Vernal pools are one of Oregon’s rarest wetland types. They form on impervious basalt bedrock or on soils cemented by a calcareous or siliceous hardpan that impedes drainage. Pools can range in size from an acre (0.4 hectares) or more to patches as small as 10-225 square feet (1-5 square meters), and may occur singly or in groups covering many acres. They usually fill with water in the fall or winter and dry up in spring or early summer, but seasonal precipitation may be highly variable and pools may fill for only brief periods or not at all. They are home to a large variety of plants and animals adapted to these harsh conditions, and some species are rare globally. Vernal pools are threatened primarily by urban or suburban development on the typically flat and easily accessible landforms in which they occur.
Wet meadows occur along the hydrologic gradient between marshes, fens, and uplands. They are seasonally moist to wet, but surface water is absent except in streams and water bodies, and many sites are dry by midsummer. Wet meadows are typically open and dominated by a diverse mix of grasses, sedges, and forbs. Patches of shrubs and trees are often present on elevated microsites or around the edges of the meadows. Fens, freshwater marshes, and lakes or ponds also are often present and intermixed with montane meadows, but the meadows are usually peripheral to wetter types and form the Eco tone to dry upland forest. Limited organic matter accumulates in wet meadows because seasonal drying allows oxidation of this material. Historically, most montane wet meadows were grazed intensively by sheep, cattle, and horses, and the drier portions were often severely degraded by soil compaction, denudation, erosion, and subsequent invasion of exotic species. Many of these sites still show the effects of overgrazing 80 or more years after they were closed to livestock.
Wet prairies are one of Oregon’s rarest wetland types. Most occur at lower elevations on bedrock or clay soils that have a seasonally perched water table. These sites usually dry out by late spring but depressions may retain water well into the summer. Wet prairie in the Willamette and Umpqua valleys are habitat to several rare species of plants. Although best known for tufted hairgrass (Deschampsia caespitosa), wet prairies contain many other species of grasses, sedges and herbs. Before flood control, wet prairies on the Columbia River bottoms were flooded for one or two months every year during the annual “spring freshet” fueled by snowmelt in the Columbia Basin. Most wet prairies have been drained, farmed, or overrun by exotic reed canary grass (Phalaris arundinacea).