Wetlands and Riparian Areas

Wetlands are ecosystems that are at least periodically saturated or inundated by water, creating unique habitats that support a wide variety of plant and animal species. Colorado wetlands include a diverse range of ecosystem types, each with distinctive plants and animals, hydrologic regimes, and ecological functions. In addition to providing habitat for wildlife and supporting biodiversity in other ecosystems, wetlands help filter water by trapping pollutants and offer a buffer zone for extreme events such as flooding.

Types of Wetlands

Wetlands in Colorado can be divided into five types: fens, marshes, wet meadows, riparian wetlands, and salt flats.

Fens have peat (undecomposed organic matter) soils and stable, groundwater-driven hydrologic regimes that limit the decomposition of organic matter. Vegetation is dominated by herbaceous plants such as sedges (Carex spp.), but mosses, shrubs, and trees can also grow. Fens commonly occur as part of wetland complexes that include riparian wetlands or wet meadows. Fens occur throughout Colorado in mountain valleys with higher precipitation and perennial inflow of groundwater from local aquifers. An especially significant site is High Creek Fen in South Park, now protected by the Nature Conservancy. The well-studied Big Meadows in Rocky Mountain National Park is also a good example.

Wet meadows have mineral soils and are typically dominated by herbaceous plants. They are the most widespread wetland type in Colorado, occurring from the plains to the alpine. Wet meadows have seasonally saturated soils but lack the perennial high water tables of fens or the large water-level fluctuations of marshes. Some wet meadows are managed for hay production and frequently develop downslope of unlined irrigation canals in agricultural areas. They may also form as marshes or beaver ponds fill with sediment. Wet meadows commonly occur in intermediate landscape positions between fens or marshes and uplands. Wet meadows can be found throughout Colorado, but a good example is Moraine Park in Rocky Mountain National Park.

Marshes have mineral soils and highly variable hydrological regimes with deep standing water occurring for extended periods. This limits plants to species tolerant of inundation, and vegetation often exhibits distinct patterns related to differences in individual species’ tolerance of flooding. Water depth and salinity are key factors determining the species composition, both within and among marshes and from wet to dry years. Water chemistry is highly variable, and marshes formed in basins where water is lost primarily through evapotranspiration can be highly saline. Marshes are critical habitat for a variety of wildlife, and waterfowl groups have actively promoted their conservation and creation. Marshes occur from the plains to the alpine and include such diverse wetland types as the playa wetlands in southeast Colorado and the numerous marshes that fringe lakes and form in abandoned beaver ponds in the mountains.

Salt flats are common at low elevations in intermountain basins, valleys, and on the plains. They can form in closed basins where water evaporates, leaving salts behind, often in sites with clay-rich soils. Plant cover and productivity is low, and communities are dominated by salt-tolerant species (halophytes). Notable salt flats occur in the southern part of South Park near Antero Reservoir—originally called “Valle Salado,” the salty bayou—as well as in the Blanca Wetlands of Alamosa County, but examples occur throughout the state.

Riparian ecosystems range from narrow communities along headwater streams in the mountains to broad alluvial rivers on the plains and western valleys. Riparian areas are influenced by unidirectional, flowing water capable of eroding and transporting sediment and are strongly shaped by the frequency, magnitude, and energy of floods. Differences in watershed size, topography, and climatic regime create varying flood regimes. Dominant riparian species include trees such as cottonwood (Populus spp.) and willows (Salix spp.). Major riparian areas in Colorado include the Cache la Poudre, South Platte, Colorado, and Arkansas Rivers, but riparian communities occur along the state’s innumerable smaller streams as well.

Ecosystem Services

Wetlands occupy about 2 percent of Colorado’s land area, but they are key landscape elements because they provide critical ecosystem services. Wetlands filter water and trap pollutants, and because of their importance to water quality, most wetlands are regulated by the federal government under section 404 of the Clean Water Act. In agricultural and urban settings, wetlands can remove excess fertilizers that degrade water quality. Riparian areas may act to buffer hydrologic extremes such as floods and drought, provide habitat for wildlife, and contribute to local biodiversity by supporting the health of adjacent aquatic and upland ecosystems.

Physical Geography

Climate, geology, and hydrologic regime are key factors influencing the formation of wetlands and riparian ecosystems. The abundance and geographic distribution of wetlands varies among Colorado’s physiographic regions and along the state’s broad elevation gradients. Because mountains receive more precipitation and are cooler than basins and lowlands, there is more water to support wetland development. Wetlands increase in abundance with elevation up to the subalpine zone but are less abundant in the alpine zone. The distribution of individual wetland types is variable. For example, fens are rare at low elevations but relatively abundant at higher elevations, while the reverse is true for salt flats.

Ecological Drivers

Hydrologic processes control many wetland and riparian functions. For example, the establishment of many riparian plants is linked to the frequency and magnitude of flooding. In marshes, the depth and duration of inundation is a key control on species composition, while seasonal water table dynamics control carbon accumulation rates in fens.

A wetland’s hydrologic regime affects nutrient cycling, plant productivity, decomposition, and the composition of plant communities. On a small scale the hydrologic regime operates as a driver of ecological structure and function, while on a broader scale it shapes patterns of wetland abundance and distribution. Wetlands can occur in basins or on slopes where there are seeps or springs capable of saturating soils, and different types of wetlands commonly co-occur as wetland complexes.

The distribution and ecology of wetlands differs between the tectonically active mountains and the inactive plains. For instance, many mountain wetlands occur in landforms produced by retreating Pleistocene glaciers. Floodplain ecosystems are dynamic, as demonstrated by the September 2013 Front Range floods, which both destroyed riparian vegetation and created conditions for new riparian plant establishment.

Wetlands and riparian areas support a variety of plant species and community types found nowhere else in Colorado. They can be dominated by only a few clonal species or be species-rich. The critical role of wetlands for biodiversity stems in part from the functional diversity of wetlands. Colorado wetlands support numerous animal species, from obligate species such as beaver (Castor canadensis) to the numerous seasonal or occasional visitors such as migratory birds and amphibians that rely on wetlands for some portion of their life cycle.

Past, Present, and Future

Since the arrival of Euro-American settlers, nearly half of Colorado wetlands have been lost due to factors such as drainage for agriculture, construction of dams, removal of beaver, the introduction of non-native species, and extensive livestock grazing. Altered fire regimes resulting from land use changes and fire suppression have also indirectly impacted wetlands.

Water storage, water diversion from streams, and riparian forest clearing have affected many riparian areas, especially along large rivers, and groundwater pumping for irrigation purposes has also affected wetlands. Other impacts stem from the alteration of stream channels and fluctuations in sediment production or transport. In addition, altered flood regimes, such as those produced by dams, can reduce opportunities for the establishment of native species and favor the spread of non-natives.

Early Euro-Americans grazed sheep in many high mountain ranges, impacts from which are still evident in some areas. Overgrazing by livestock or native ungulates such as elk or moose can negatively impact wetlands and riparian areas, wildlife and fish habitat, and the regeneration of native plants.

Non-native species are a significant threat to many areas. For example, woody species like salt cedar (Tamarix spp.) and Russian-olive (Elaeagnus angustifolia) were introduced to North America from Europe and Asia and are highly invasive in riparian areas at lower elevations. Many non-native species respond positively to disturbance and are associated with roads and other disturbed environments.

Placer mining affected many mountain streams, such as those near Breckenridge and Fairplay, while wetlands along rivers at lower elevations have been impacted by sand and gravel mining. Hard rock mining has produced runoff contaminated with metals. Oil, natural gas, and coal extraction can alter wetlands through hydrological changes associated with roads and well pads.

Roads are commonly built along valley bottoms, and many streams have been channeled and the wetlands filled. Hydrologic function can also be impaired by culverts and ditches, which alter drainage patterns.

Wetlands and riparian areas are likely to experience significant changes in the future from climate change and human population growth. Shifts in the timing of precipitation, the proportion of precipitation occurring as rain versus snow, and changes in broad-scale weather phenomena like the Southwest monsoon will be especially important. Scientists continue to advance our understanding of how Colorado wetlands function and the factors influencing their condition. While federal regulations provide some protection for wetlands, wetlands remain vulnerable to human impacts. Efforts by wetland scientists and conservationists are underway to identify and preserve ecologically important wetlands and to develop improved techniques for restoring degraded ecosystems and the critical ecosystem services they provide.