Local freshwater wetlands are diverse and abundant throughout the Columbia Coast. These habitats include fens, which are complexes of swales and dunes with a variety of wetlands in the swales, swamps, shrublands, marshes, sphagnum, seasonal wetlands, wet prairies, riparian habitats, seeps and springs and wet rock slopes. A few true bogs with isolated hydrology can also be found on upland sites. Though local usuage calls both sphagnum marshes and cranberry farms "bogs," neither wetland is isolated from groundwater or a true bog in the hydrological sense.

Fens are dune-swale complexes, found on Clatsop Plains, Long Beach Peninsula and Twin Harbors beaches. These wetlands are hydrologically connected by groundwater from swale to swale. Common wetland species include Nootka reedgrass, slough sedge, Hooker's Willow, Douglas's aster, western crabapple, and red alder. Fen complexes contain marshes, open water areas, swamps, seasonal wetlands and adjacent upland vegetation. In these areas, upland vegetation goes through succession from dune grassland to shore pine forest to Sitka spruce forest to western hemlock. (See Swamps for details).

Water levels in the fens have been severely altered in the past century, resulting in lower winter water levels and drier summer surfaces. In addition, filling for development fragmented and isolated many wetlands, impeding drainage, and with agriculture and urban development, virtually eliminated native plant communities by displacement with cultivated non-native species and weeds.

Shrublands are too wet to grow conifers, but support a variety of shrubs, including willows, sweet gale, Douglas's spirea, western crabapple, bog Labrador tea, bog laurel, and sedges. They are often important habitats for migratory warblers and amphibians.

Marshes are dominated by grasses, sedges and rushes, with some shrubby species. They represent either a transitional stage between young interdunal wetlands and coastal forest, or a permanently wet site, so wet that trees cannot establish. Common species include slough sedge, Sitka sedge, common or soft rush, ladyfern, Douglas spirea, sweet gale and western crabapple. Spirea and willows may grow throughout marshes, or only on the edges. In swales along the ocean, shore pine and willows rapidly invade marshes, beginning the transition to swamps.

Sphagnum-dominated marshes are rare, usually found in older marsh sites, with low growing acid-tolerant plants. Bog Labrador tea, sedges, creeping bunchberry, and sundew are common plants. Floating bogs are found in a few places. These marshes tend to have high species diversity, and to be restricted to very wet, old marsh sites, such as on the eastern borders of fens north and south of the Columbia River. Many low marshes were destroyed in the past century by changes in hydrology.

A few true bogs can be found on the Columbia Coast. These sites are hydrologically isolated from groundwater by geology, and are dominated by sphagnum moss and acid-tolerant flowering plants, very similar in composition to sphagnum marshes in fens.

Seasonal wetlands / vernal pools are common along the coast in swales. These interdunal wetlands, with low growing plants and some open soil, may more correctly be called vernal pools. They tend to have standing water in winter, and to be dry in summer. Common species include a variety of rushes, creeping spearwort, veronica, and cinquefoil. They are short-lived habitats that soon vegetate with dense stands of sedges, which in turn are invaded by willows, shore pine and other species of coastal swamps.

Wet prairies are low marshes, found on open, sunny inland sites in river valleys. Often highly disturbed by ditching and draining for agriculture, they are floristically related to similar sites in the Willamette - Puget Trough. Sedges, grasses and a group of unique wetland herbs are found in these prairies.

Wet rocky slopes can be sites for showy annuals and perennials, including Nuttall's saxifrage, frigid shooting star, small-leaved montia, western saxifrage. Wet slopes on Saddle Mountain grow a wide variety of species.

Seeps and springs are found on open sunny slopes and in deep shade. Sunny, open wet sites often have many flowering species, while those deep in forest shade will have mosses, liverworts and ferns.

Riparian areas are above the reach of tides, and include river and stream edges. They grow a wide variety of plants depending on substrate and light conditions. Cattails, small-seeded bulrush, slough sedge and canary reedgrass are common in slower moving waters. Soft rush, docks, Pacific water parsley

Aquatic habitats are permanently wet, with free-floating and rooted aquatic plants. Species include watershield, yellow pond-lily, several pondweeds, smartweeds, duckweed, cattails, bur-reed, and several aggressive non-natives, including Brazilian elodea (now Egeria), purple loosestrife, crisp pondweed and Eurasian watermilfoil. Many of these species are also found in backwaters of the Columbia and other rivers.