The Western North American Emergent Marsh system includes wetlands with permanent to semi-permanent standing water that support aquatic, submerged, and coarse emergent plants. For Colorado, this system is inclusive of both the concept of the North American Arid West Emergent Marsh as defined by NatureServe, as well as higher-elevaiton marshes in the Rocky Mountains, Colorado intermountain valleys, and other habitats that have greater annual precipitation than the arid west. Natural marshes may occur in depressions (impounded ponds or kettle ponds), on lake fringes, or within riparian and floodplain areas as beaver ponds, backwater channels, oxbows, or sloughs. Marshes are frequently or continually inundated, with water depths up to 2 m. Water levels may be stable, or may fluctuate 1 m or more over the course of the growing season. Hydrologic inputs include direct precipitation, surface water inflows, and groundwater discharge. Marshes have distinctive soil characteristics that result from long periods of anaerobic conditions in the soils (e.g., gleyed soils, high organic content, redoximorphic features). The vegetation is characterized by herbaceous plants that are adapted to saturated soil conditions. Common emergent and floating vegetation includes species of bulrush (Scirpus and/or Schoenoplectus), cattail (Typha), rush (Juncus), pondweed (Potamogeton), smartweed (Polygonum), pondlily (Nuphar), and canarygrass (Phalaris). This system may also include areas of relatively deep water with floating-leaved plants such as duckweed (Lemna), and submerged and floating plants such as watermilfoil (Myriophyllum), hornwort (Ceratophyllum), and waterweed (Elodea). Marsh vegetation is occasionally bordered by woody species such as plains cottonwood (Populus deltoides) and willows (Salix spp.).

Marshes are permanent or semi-permanent wetlands with relatively deep standing water (>20 cm and up to 2 m). Marsh vegetation is typically dominated by coarse emergent and/or aquatic plant species. Soils are mineral and typically gleyed or reduced, but may have a high organic content in the A horizon.

This system occurs throughout much of the arid and semi-arid regions of western North America. In Colorado, it occurs throughout the state from the plains to upper elevations. There are particular concentrations along the Front Range, the South Platte and Arkansas River floodplains, and the San Luis Valley.

Western North American Emergent Marsh is a small patch system.

Marshes occur throughout the entire state of Colorado, occupying a wide range of habitats from depressions, lake fringes, ponds, old channels, oxbows, and sloughs. In the plains, marshes occur along slow-moving streams, within backwater areas of the major floodplains, as well as along reservoir fringes. Within the Rocky Mountains, glacial and fluvial processes have sculpted kettle ponds and other depressions where marsh systems develop. Emergent marshes are also frequently found associated with excess runoff and elevated groundwater tables associated with irrigated agriculture or stormwater runoff. Marsh vegetation thrives wherever the water is impounded and above the ground surface for extended periods.

Marsh hydrology is supported primarily by precipitation and surface water runoff and is particularly responsive to climatic fluctuations. Precipitation and evapotranspiration can determine water level fluctuation as well as soil chemistry due to evaporative salts in the soil. Marshes can have up to 2 m of standing water above the soil surface and fluctuation of the water table in marshes can be large (> 1 m). During drought years, many marshes may be significantly drier than during normal precipitation years. Water sources can also include groundwater, stream discharge, and overbank flow.

Marshes have distinctive soils that are typically mineral, but can also accumulate organic material depending on the duration of inundation and frequency of fluctuation. Because the water table in marshes fluctuates, the soil is periodically aerated allowing organic matter to decompose, preventing the accumulation of peat in many marshes. Soils often possess characteristics that result from long periods of anaerobic conditions in the soils such as gleyed soils, high organic content, and redoximorphic features.

Marsh vegetation is characterized by permanent to semi-permanently and seasonally flooded herbaceous vegetation types. Plant species are dominated by coarse emergent perennial and often rhizomatous graminoids. Vegetation height can range from 0.5-5 m. Cover can be dense or open, depending on location and management regime. Floating and submerged aquatic species are also often present. The forb component is typically sparse, ranging from 0-25% cover.

Emergent marshes can have distinct zonation according to the degree of inundation and soil moisture, and can even grade into the drier wet meadow systems. The most characteristic species include coarse emergents such as cattail (Typha spp.), bulrush (Schoenoplectus spp., Bolboschoenus spp.), arrowhead (Sagittaria spp.), sweet coltsfoot (Petasites sagittata). Marshes with deep open water often contain true aquatic species such as duckweed (Lemna spp.), pond lily (Nuphar lutea), pondweeds (Potamogeton spp.); watermilfoil (Myriophyllum), hornwort (Ceratophyllum), and waterweed (Elodea). Within the shallower edges, finer emergent species grow such as spikerush (Eleocharis spp.), sedges (Carex spp.), rushes (Juncus spp.), cordgrass (Spartina spp.), cutleaf water parsnip (Berula erecta), American speedwell (Veronica anagallis-aquatica), and roundleaf monkey flower (Mimulus glabratus). Marshes with a transitional zone from moist soil to upland may include drier sedges, rushes, tufted hairgrass (Deschampsia cespitosa), bearded sprangletop (Leptochloa fusca), reed canarygrass (Phalaroides arundinacea), foxtail barley (Critesion jubatum), saltgrass (Distichlis stricta), showy milkweed (Asclepias speciosa), wild mint (Mentha arvensis), mapleleaf goosefoot (Chenopodium simplex), and curlytop knotweed (Persicaria lapathifolia). In some marshes, woody species will border the inundated zones including plains cottonwood (Populus deltoides), narrowleaf willow (Salix exigua), peachleaf willow (Salix amygaloides), Western snowberry (Symphoricarpos occidentalis), and noxious species, Russian olive (Elaeagnus angustifolia) and saltcedar (Tamarix ramosissima). Not all of these zones are present in every marsh, as shoreline gradient and hydrological regime varies between region and wetland.

In Colorado, marshes and ponds support high priority bird species. Although native marshes have suffered extensive loss and have changed from native condition, they still provide key habitat for avifauna. In Colorado priority bird species that depend on wetlands for breeding include Northern Harrier (Circus cyaneus), Short-eared Owl (Asio flammeus) and Willet (Tringa semipalmata). Other marsh associated species include American Bittern (Botaurus lentiginosus), Black Rail (Laterallus jamaicensis), Virginia Rail (Rallus limicola), Sora (Porzana carolina), Common Snipe (Gallinago delicata), Marsh Wren (Cistothorus palustris), Red-winged Blackbird (Agelaius phoeniceus), Yellow-headed Blackbird (Xanthocephalus xanthocephalus), Wilson's Phalarope (Phalaropus tricolor), American Avocet (Recurvirostra americana), Killdeer (Charadrius vociferous), Savannah Sparrow (Passerculus sandwichensis) and a variety of waterfowl.

Marshes also provide critical habitat for a variety of herptofauna, including uncommon and rare species. Mountain marshes provide habitat for species such as tiger salamander (Ambystoma tigrinum) (ubiquitous), mountain toad (Bufo boreas) (formerly widespread now scarce), western chorus frog (Pseudacris triseriata) (ubiquitous), western terrestrial garter snake (Thamnophis elegans) (ubiquitous) and wood frog (Rana sylvatica). Lowland marshes provide habitat for tiger salamander, Woodhouse's toad (Bufo woodhousii), western chorus frog, plains and northern leopard frogs (Rana blairi and R. pipiens), bullfrog (Rana catesbeiana)(non-native), snapping turtle (Chelydra serpentina), painted turtle (Chrysemys picta), northern water snake (Nerodia sipedon), bullsnake (Pituophis catenifer) and garter snakes (Thamnophis spp.).

Climate and basin morphology are the primary ecological drivers of marsh vegetation patterns and ecological function through their effects on hydrology. All natural wetland functions are closely related to the water cycle with hydroperiod as the principle factor that controls wetland structure and function. Marshes are usually associated with a permanent water source, and have a water table close to the surface that persists for one or more months during the growing season. Hydrologic inputs include direct precipitation, surface water inflows, and groundwater discharge. Outflows can occur as surface water, direct evaporation, plant transpiration, or percolation into groundwater aquifers.

The hydrologic regime affects the rates of productivity, nutrient cycling, and species composition. The seasonality, depth, and, duration of inundation influence marsh function and vegetation dynamics and vary widely among different wetland types and within an individual class of wetlands such as marshes. Marsh development along riparian areas is driven by the magnitude and frequency of flooding, valley and substrate type, and beaver activity. Seasonal and episodic flooding scour depressions in the floodplain, create side channels and floodplain sloughs, and force channel migration which can result in oxbows. Marsh vegetation establishes in these landforms if semi-permanent to permanent water is adequate and available. Marshes also develop near the fringes of lakes and ponds where their occurrence is dependent on the gradient of the shoreline and fluctuation of lake or pond levels. Relatively flat or gently sloping shorelines support a much larger marsh system than a steep sloping shoreline. The frequency and magnitude of water level fluctuations determine the extent of each marsh zone (floating, submerged, emergent, etc.).

Climatic conditions and management of surrounding land and water sources will impact marshes and their plant communities. Impacts of global climate change are altering the amount, form, and seasonality of precipitation that maintain riparian and marsh wetland systems. Localized human management is also a major issue for marshes as they are intimately connected to the uplands in their watersheds therefore draining, ditching and conversion of land for crops or grazing will alter they hydrologic regime. Grazing, logging, upland development and mining can introduce excess nutrients, pollutants, and sediments in runoff and degrade marsh integrity. Land disturbance such as heavy grazing, agriculture or change in water regimes may lead to invasion by many non-native species including purple loosestrife (Lythrum salicaria), barnyard grass (Echinochloa crus-galli), and reed canarygrass (Phalaris arundinacea). Pasture grasses such as Kentucky bluegrass (Poa pratensis), redtop (Agrostis gigantea), and timothy (Phleum pratense) as well as exotics species common to other wetland types such as Canada thistle (Cirsium arvense) and dandelion (Taraxacum officinale) may invade marsh margins. Also, dense stands of native plants such as broad-leaf cattail (Typha latifolia) and common reed (Phragmites australis) chokes out other native vegetation and makes animal movement impossible causing degradation of habitat and function in marshes. Land managers often implement removal of these monocultures to open up resources and space for plants and animals.