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Montana Field Guides

Great Plains Open Freshwater Depression Wetland

Provisional State Rank: S3

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General Description

This Great Plains system occurs in lowland depressions and along lake borders with open basins and a permanent water source through most of the year. This system is distinguished from the Western Great Plains Closed Depression Wetlands by having a larger watershed and/or significant connection to the groundwater table. Soils are typically Mollisols, Entisols or occasionally Histosols. Soil pH varies from neutral to slightly alkaline. In Montana, this system is especially well represented along major and secondary tributaries of the Milk, Marias and Two Medicine rivers in the northwestern Great Plains glaciated pothole region. Throughout Montana, most sites within this system are found at elevations of 664-2,027 meters (2,180-6,650 feet). Species diversity can be high in some occurrences. These wetlands usually contain emergent graminoids such as cattails (Typha species), sedges (Carex species), spikerushes (Eleocharis species), rushes (Juncus species) and bulrushes (Schoenoplectus species), as well as floating vegetation such as pondweeds (Potamogeton species), arrowhead (Sagittaria species), or common hornwort (Ceratophyllum demersum). At montane elevations, these systems can be moderately complex with a variety of species and communities. Increased grazing pressure in and adjacent to these systems will change the plant communities that are present. In semi-permanent systems, the drawdown zone is typically dominated by beaked sedge (Carex utriculata) water sedge (Carex aquatilis), and Nebraska sedge (Carex nebrascensis). In seasonal ponds that draw down annually, and in semipermanent wetlands during drought years, buried seeds of both annuals and perennials will germinate in exposed mud flats.

Diagnostic Characteristics
Herbaceous, depression, depressional, saturated soils, partially isolated

Similar Systems

This system occurs across the western Great Plains from North Dakota and Kansas west to Montana and south to Texas. This system can occur throughout the western Great Plains but is likely more prevalent in the south-central portions of the division. Its distribution extends into central Montana, where it occurs in the matrix of the Northwestern Great Plains Mixed Grass Prairie. However, these depressions are most concentrated to the north of the Hi-Line and Route 2, from the Blackfeet Reservation to the North Dakota border. Individual depressions can also be found across the Northwest Glaciated Plains north of the Missouri River.

Ecological System Distribution
Approximately 138 square kilometers are classified as Great Plains Open Freshwater Depression Wetland in the 2016 Montana Land Cover layers.  Grid on map is based on USGS 7.5 minute quadrangle map boundaries.

Montana Counties of Occurrence
Big Horn, Blaine, Carbon, Carter, Cascade, Chouteau, Daniels, Fallon, Fergus, Garfield, Glacier, Golden Valley, Hill, Judith Basin, Lewis and Clark, Liberty, Mccone, Musselshell, Park, Petroleum, Phillips, Pondera, Prairie, Richland, Roosevelt, Rosebud, Sheridan, Stillwater, Sweet Grass, Teton, Toole, Treasure, Valley, Wheatland, Yellowstone

Spatial Pattern


Open depression wetlands are found throughout the Northwestern Glaciated Great Plains region of Montana. They form in lowlands, and along lake borders and stream margins. They generally have more open basins, a large watershed, and a permanent water source throughout most of the year, except during exceptional drought years. This system differs from closed depressional wetlands by having a larger watershed and/or significant connection to the groundwater table (Cook and Hauer 2007). In Montana, most sites within this system are found at elevations of 664-2,027 meters (2,180-6,650 feet). Soils are typically Mollisols, Entisols or occasionally Histosols. Soil pH varies from neutral to slightly alkaline.


Open depression wetlands often have submerged aquatic plants in the open water zone including common hornwort (Ceratophyllum demersum), short spikewater milfoil (Myriophyllum sibiricum), and horned pondweed (Zannichellia palustris) as well as floating-leaved plants including  pondweeds (Stuckenia and Potamogeton species), white water crowfoot (Ranunculus aquatilis) and arrowheads (Sagittariaspecies).  The central marsh zone is typically dominated by hardstem bulrush (Schoenoplectus acutus), but softstem bulrush (Schoenoplectus tabernaemontani), common threesquare (Schoenoplectus pungens) and alkali bulrush (Schoenoplectus maritimus), often co-dominate. Also found in the marsh zone are cattails (Typha species), water knotweed (Polygonum amphibium), and hemlock water parsnip (Sium suave).  The seasonally flooded zones are typically dominated by graminoids including common spikerush (Eleocharis palustris), needle spikerush (Eleocharis acicularis), American sloughgrass (Beckmannia syzigachne), wheat sedge (Carex atherodes), foxtail barley (Hordeum jubatum), shortawn foxtail (Alopecurus aequalis), and water foxtail (Alopecurus geniculatus).  Open depressional systems are often bordered by wet prairie zones characterized by species such as slimstem reedgrass (Calamagrostis stricta), clustered field sedge (Carex praegracilis), bluejoint (Calamagrostis canadensis) and fowl bluegrass (Poa palustris). Open depressions with more alkaline or saline water and soil chemistry will typically be bordered by species such as saltgrass (Distichlis spicata), western wheatgrass (Pascopyrum smithii), and freshwater cordgrass (Spartina pectinata).  Sites that have been moderately grazed often have an increase in Baltic rush (Juncus balticus), knotted rush (Juncus nodosus), foxtail barley (Hordeum jubatum), American sloughgrass (Beckmannia syzigachne), and western wheatgrass (Pascopyrum smithii).

Alliances and Associations
  • (A.1403) (Beaked Sedge, Northwest Territory Sedge) Seasonally Flooded Herbaceous Alliance
  • (A.1422) (Common Spikerush, Page Spikerush) Seasonally Flooded Herbaceous Alliance
  • (A.1436) (Narrowleaf Cattail, Broadleaf Cattail) - (Clubrush species) Semipermanently Flooded Herbaceous Alliance
  • (A.1436) (Narrowleaf Cattail, Broadleaf Cattail) - (Clubrush species) Semipermanently Flooded Herbaceous Alliance
  • (A.1404) Aquatic Sedge Seasonally Flooded Herbaceous Alliance
  • (A.1396) Awned Sedge Seasonally Flooded Herbaceous Alliance
  • (A.1374) Baltic Rush Seasonally Flooded Herbaceous Alliance
  • (A.1675) Broadleaf Arrowhead Semipermanently Flooded Herbaceous Alliance
  • (A.1394) Cattail species - (Clubrush species, Rush species) Seasonally Flooded Herbaceous Alliance
  • (A.1342) Common Spikerush Temporarily Flooded Herbaceous Alliance
  • (A.1443) Hardstem Bulrush - (Softstem Bulrush) Semipermanently Flooded Herbaceous Alliance
  • (A.1417) Nebraska Sedge Seasonally Flooded Herbaceous Alliance
  • (A.1754) Pondweed species - Coontail species - Waterweed species Permanently Flooded Herbaceous Alliance
  • (A.1347) Prairie Cordgrass Temporarily Flooded Herbaceous Alliance
  • (A.1765) Red-head Pondweed Permanently Flooded Herbaceous Alliance
  • (A.1381) Reed Canarygrass Seasonally Flooded Herbaceous Alliance
  • (A.1764) Sago Pondweed Permanently Flooded Herbaceous Alliance
  • (A.1350) Sedge species - Alkali Plantain Temporarily Flooded Herbaceous Alliance
  • (A.1465) Sedge species - Cattail species Saturated Herbaceous Alliance
  • (A.1455) Sedge species Saturated Herbaceous Alliance
  • (A.1445) Small Floating Mannagrass Semipermanently Flooded Herbaceous Alliance
  • (A.1348) Smartweed species - Barnyard Grass species Temporarily Flooded Herbaceous Alliance
  • (A.1881) Smartweed species Seasonally Flooded Herbaceous Alliance
  • (A.1401) Sprangletop Seasonally Flooded Herbaceous Alliance
  • (A.1354) Western Wheatgrass Temporarily Flooded Herbaceous Alliance

Dynamic Processes
These systems developed under Northern Great Plains climatic conditions, and experienced the natural influence of large herbivores, periodic flooding events and occasional fire. Wet-drought year climatic cycles in Montana, often in 10 to 20 year intervals, influence the ecological communities (Hansen et al., 1996). Seeds from annuals and perennials germinate and cover exposed mud flats, but when precipitation floods the depressions, the annuals drown and the perennials survive. Over a series of years the perennials dominate. The drawdown to mudflats is necessary so that emergent vegetation can become reestablished. Flooding, drawdown and the eventual exposure of mud flats drive the water-level vegetation cycle. Species richness can vary considerably among individual examples and is especially influenced by adjacent land use. Agriculture may provide nutrient and herbicide runoff. In saline soil wetlands, the increase in precipitation during exceptionally wet years can dilute the salt concentration in the soils, which may allow for less salt-tolerant species to occur.

Changes will occur in the plant communities due to climatic conditions and/or management activities. Conversion to agriculture and pastureland can impact this system when it alters the hydrology of the system.

Restoration Considerations
In open depression wetland systems where water has been drained or diverted, the original hydrology of the system must be restored. If water levels are restored, re-growth and re-colonization from dormant rhizomatous root systems of common emergent species can occur within a few years. Livestock grazing should be controlled to allow regrowth, recolonization and resprouting from existing root systems. Many of the characteristic species found in these systems are rhizomatous, and exhibit excellent erosion control properties. In some cases, if hydric soils are heavily altered due to pugging or compaction, addition of organic material may be needed to facilitate vegetation recolonization.

Species Associated with this Ecological System
  • Details on Creation and Suggested Uses and Limitations
    How Associations Were Made
    We associated the use and habitat quality (common or occasional) of each of the 82 ecological systems mapped in Montana for vertebrate animal species that regularly breed, overwinter, or migrate through the state by:
    1. Using personal observations and reviewing literature that summarize the breeding, overwintering, or migratory habitat requirements of each species (Dobkin 1992, Hart et al. 1998, Hutto and Young 1999, Maxell 2000, Foresman 2012, Adams 2003, and Werner et al. 2004);
    2. Evaluating structural characteristics and distribution of each ecological system relative to the species' range and habitat requirements;
    3. Examining the observation records for each species in the state-wide point observation database associated with each ecological system;
    4. Calculating the percentage of observations associated with each ecological system relative to the percent of Montana covered by each ecological system to get a measure of "observations versus availability of habitat".
    Species that breed in Montana were only evaluated for breeding habitat use, species that only overwinter in Montana were only evaluated for overwintering habitat use, and species that only migrate through Montana were only evaluated for migratory habitat use.  In general, species were listed as associated with an ecological system if structural characteristics of used habitat documented in the literature were present in the ecological system or large numbers of point observations were associated with the ecological system.  However, species were not listed as associated with an ecological system if there was no support in the literature for use of structural characteristics in an ecological system, even if point observations were associated with that system.  Common versus occasional association with an ecological system was assigned based on the degree to which the structural characteristics of an ecological system matched the preferred structural habitat characteristics for each species as represented in scientific literature.  The percentage of observations associated with each ecological system relative to the percent of Montana covered by each ecological system was also used to guide assignment of common versus occasional association.  If you have any questions or comments on species associations with ecological systems, please contact the Montana Natural Heritage Program's Senior Zoologist.

    Suggested Uses and Limitations
    Species associations with ecological systems should be used to generate potential lists of species that may occupy broader landscapes for the purposes of landscape-level planning.  These potential lists of species should not be used in place of documented occurrences of species (this information can be requested at: or systematic surveys for species and evaluations of habitat at a local site level by trained biologists.  Users of this information should be aware that the land cover data used to generate species associations is based on imagery from the late 1990s and early 2000s and was only intended to be used at broader landscape scales.  Land cover mapping accuracy is particularly problematic when the systems occur as small patches or where the land cover types have been altered over the past decade.  Thus, particular caution should be used when using the associations in assessments of smaller areas (e.g., evaluations of public land survey sections).  Finally, although a species may be associated with a particular ecological system within its known geographic range, portions of that ecological system may occur outside of the species' known geographic range.

    Literature Cited
    • Adams, R.A.  2003.  Bats of the Rocky Mountain West; natural history, ecology, and conservation.  Boulder, CO: University Press of Colorado.  289 p.
    • Dobkin, D. S.  1992.  Neotropical migrant land birds in the Northern Rockies and Great Plains. USDA Forest Service, Northern Region. Publication No. R1-93-34.  Missoula, MT.
    • Foresman, K.R.  2012.  Mammals of Montana.  Second edition.  Mountain Press Publishing, Missoula, Montana.  429 pp.
    • Hart, M.M., W.A. Williams, P.C. Thornton, K.P. McLaughlin, C.M. Tobalske, B.A. Maxell, D.P. Hendricks, C.R. Peterson, and R.L. Redmond. 1998.  Montana atlas of terrestrial vertebrates.  Montana Cooperative Wildlife Research Unit, University of Montana, Missoula, MT.  1302 p.
    • Hutto, R.L. and J.S. Young.  1999.  Habitat relationships of landbirds in the Northern Region, USDA Forest Service, Rocky Mountain Research Station RMRS-GTR-32.  72 p.
    • Maxell, B.A.  2000.  Management of Montana's amphibians: a review of factors that may present a risk to population viability and accounts on the identification, distribution, taxonomy, habitat use, natural history, and the status and conservation of individual species.  Report to U.S. Forest Service Region 1.  Missoula, MT: Wildlife Biology Program, University of Montana.  161 p.
    • Werner, J.K., B.A. Maxell, P. Hendricks, and D. Flath.  2004.  Amphibians and reptiles of Montana.  Missoula, MT: Mountain Press Publishing Company. 262 p.

Original Concept Authors
Natureserve Western Ecology Group

Montana Version Authors
L. Vance, C. McIntyre, T. Luna

Version Date

  • Classification and Map Identifiers

    Cowardin Wetland Classification:
    System Palustrine
    Class Emergent wetland
    Water Regime Temporarily to seasonally flooded
    Geographically Isolated Wetland Partially isolated

    National Vegetation Classification Standard:
    Class Shrubland and Grassland
    Subclass Temperate and Boreal Shrubland and Grassland
    Formation Temperate and Boreal Fresh Water Wet Meadow and Marsh
    Division Eastern North American wet Meadow, Riparian and Marsh
    Macrogroup Great Plains Freshwater Wet Meadow, Riparian and Marsh

    NatureServe Identifiers:
    Element Global ID 28532
    System Code CES303.675, Western Great Plains Open Freshwater Depression Wetland

    9218: Western Great Plains Open Freshwater Depression Wetland

  • Additional ReferencesLegend:   View Online Publication
    Do you know of a citation we're missing?
    • Cook, Bradley J., and F. Richard Hauer. 2007. "Effects of hydrologic connectivity on water chemistry, soils, and vegetation structure and function in an intermontane depressional wetland landscape". Wetlands. 27 (3): 719-738.
    • Hansen, P. L., R. D. Pfister, K. Boggs, B. J. Cook, J. Joy, and D. K. Hinckley. 1995. Classification and management of Montana's riparian and wetland sites. Montana Forest and Conservation Experiment Station, School of Forestry, University of Montana, Miscellaneous Publication No. 54. 646 pp. + posters.

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Citation for data on this website:
Great Plains Open Freshwater Depression Wetland — Western Great Plains Open Freshwater Depression Wetland.  Montana Field Guide.  Retrieved on , from