Ecoregions of Montana (EPA)

Source: EPA

caption Ecoregions of Montana. (Map source: USEPA)

The second edition of "Ecoregions of Montana" revises many ecoregion polygon assignments that appeared in the first edition (Woods and others, 1999). These changes were made after research in Idaho (McGrath and others, 2002) recognized the Idaho Batholith as a separate level III ecoregion (Ecoregion 16), limited the Northern Rockies (15) to strongly marine-influenced areas, and transferred the Montana Valley and Foothill Prairies (formerly Ecoregion 16) to another level III ecoregion, the Middle Rockies (17). The second edition also modifies a few level IV ecoregion lines along Montana's western border so that ecoregions shared by Montana and Idaho will edge match. In addition, it updates ecoregion names so that they are consistent with the most recent ecoregion work in area (Chapman and others, 2003). However, it is important to note that although many polygon assignments and a few ecoregion names have changed between the first and second editions, nearly all level IV ecoregion line positions are identical on the two editions.

Ecoregions denote areas of general similarity in ecosystems and in the type, quality, and quantity of environmental resources; they are designed to serve as a spatial framework for the research, assessment, management, and monitoring of ecosystems and ecosystem components. Ecoregions are directly applicable to the immediate needs of state agencies, including the development of biological criteria and water quality standards and the establishment of management goals for nonpoint-source pollution. They are also relevant to integrated ecosystem management, an ultimate goal of most federal and state resource management agencies.

caption Level III Ecoregions of the Conterminous United States. (Map source: USEPA, 2000)

The approach used to compile this map is based on the premise that ecological regions can be identified through the analysis of the spatial patterns and the composition of biotic and abiotic phenomena that affect or reflect differences in ecosystem quality and integrity (Wiken 1986; Omernik 1987, 1995). These phenomena include geology, physiography, vegetation, climate, soils, land use, wildlife, and hydrology. The relative importance of each characteristic varies from one ecological region to another regardless of the hierarchical level. A Roman numeral hierarchical scheme has been adopted for different levels of ecological regions. Level I is the coarsest level, dividing North America into 15 ecological regions. Level II divides the continent into 52 regions (Commission for Environmental Cooperation Working Group 1997). At level III, the continental United States contains 104 regions (United States Environmental Protection Agency [USEPA], 1998). Level IV is a further subdivision of level III ecoregions. Explanations of the methods used to define the USEPA’s ecoregions are given in Omernik (1995), Griffith and others (1994), and Gallant and others (1989). The level III and IV ecoregion map on this poster was compiled at a scale of 1:250,000 and depicts revisions and subdivisions of earlier level III ecoregions that were originally compiled at a smaller scale (USEPA 1998; Omernik 1987). This poster is part of a collaborative project primarily between USEPA Region VIII, USEPA National Health and Environmental Effects Research Laboratory (Corvallis, Oregon), Montana Department of Environmental Quality (MDEQ), United States Department of Agriculture-Forest Service (USFS), United States Department of Agriculture-Natural Resources Conservation Service (NRCS) (formerly Soil Conservation Service), United States Department of the Interior-Bureau of Land Management (BLM), and United States Department of the Interior-U.S. Geological Survey (USGS)-Earth Resources Observation Systems (EROS) Data Center.

The project is associated with an interagency effort to develop a common framework of ecological regions. Reaching that objective requires recognition of the differences in the conceptual approaches and mapping methodologies applied to develop the most common ecoregion-type frameworks, including those developed by the USFS (Bailey and others, 1994), the USEPA (Omernik 1987, 1995), and the NRCS (U.S. Department of Agriculture-Soil Conservation Service, 1981). As each of these frameworks is further refined, their differences are becoming less discernible. Regional collaborative projects such as this one in Montana, where agreement has been reached among multiple resource management agencies, is a step toward attaining consensus and consistency in ecoregion frameworks for the entire nation.

15. Northern Rockies
Ecoregion 15 is mountainous and rugged. Climate, trees, and understory species are characteristically maritime-influenced. Douglas-fir, subalpine fir, Englemann spruce, western larch, lodgepole pine, and ponderosa pine as well as Pacific indicators such as western redcedar, western hemlock, mountain hemlock, and grand fir occur. Pacific tree species are more numerous than in the Idaho Batholith (16) and are never dominant in the Middle Rockies (17). Alpine areas occur but, as a whole, the region has lower elevations, less perennial snow and ice, and fewer glacial lakes than the adjacent Canadian Rockies (41). Metasedimentary rocks are common; granitic rocks and associated management problems are less extensive than in the Idaho Batholith (16). Thick volcanic ash deposits are more widespread than in Ecoregion 16. Logging and mining are common and have caused stream water quality problems in the region. Recreational uses are also important.
caption The Northern Rockies (15) ecoregion is rugged and forested. Precipitation is often orographically controlled but generally decreases toward the south. The photograph was taken near Nez Perce Pass.
In Montana, Ecoregion 15 has enough variation in its biotic and abiotic characteristics to be subdivided into 12 level IV ecoregions (15a through 15e, 15h, 15k, 15l, 15o through 15q, and 15t). These level IV ecoregions typically separate areas that have different types of climate, climax vegetation, rock, water quality, terrain, and/or soil. The climate of the northwestern section of Ecoregion 15 has a strong Pacific influence. In consequence, the area has more tree species, more diverse forests, and more winter cloudiness than southeastern sections. Rainfall and snow melt water are usually plentiful in the Northern Rockies (15), especially at higher elevations. Some water is stored in unconsolidated mountain valley fill, volcanic deposits, glacial deposits, and in permeable sedimentary rocks. Ecoregion 15 in Montana is largely underlain by metamorphic rocks. These crystalline rock areas lack significant groundwater storage capacity; as a result, base flow is limited, overland runoff is common, and stream flow at high elevations is typically restricted to rain and snow melt periods.
16. Idaho Batholith
Ecoregion 16 is mountainous, deeply dissected, partially glaciated, and characteristically underlain by granitic rocks. The lithological mosaic and related slope stability and water quality issues are different from Ecoregions 15 and 17. Soils derived from granitics are droughty and have limited fertility, and therefore provide only limited amounts of nutrients to aquatic ecosystems. They are highly erodible when vegetation is removed. Douglas-fir, ponderosa pine, and, at higher elevations, subalpine fir occur. Maritime influence is slight. Pacific tree species are less numerous than in Ecoregion 15; western hemlock is absent. Overall, the vegetation is unlike that of Ecoregions 15 and 17. Land uses include logging, grazing, and recreation. Streams are likely to suffer from increased loads of fine sediments after disturbance by humans. Fish assemblage composition is similar to Ecoregion 15.

caption Typically, the Idaho Batholith (16) is underlain by granitics and is lithologically and climatically distinct from the Northern Rockies (15). High elevation areas are characterized by rockland, talus, tarns, and open high elevation forests.
In Montana, Ecoregion 16 has been broken into 4 level IV ecoregions (16a, 16b. 16e. and 16h). Each ecoregion is characterized by significant differences in terrain, climate, elevation, and/or potential natural vegetation, and aquatic habitat. Lake-studded, hummocky foothills at the mouths of glaciated canyons have biotic assemblages that are different from other less-glaciated foothills. Highest elevations are treeless, typically snow- and ice-covered, and can be lake-studded. They receive more precipitation than lower ecoregions.
17. Middle Rockies
The climate of Ecoregion 17 lacks the strong maritime influence of Ecoregion 15. Mountains have Douglas-fir, subalpine fir, and Engelmann spruce forests and alpine areas. Pacific tree species such as grand fir, western hemlock, and western redcedar are never dominant and forests can have open canopies. Foothills are partly wooded or shrub- and grass-covered. Intermontane valleys are grassland or shrub-covered and contain a mosaic of terrestrial and aquatic fauna that is distinct from nearby mountains. Many mountain-fed streams occur. Granitics and associated management problems are less extensive than in Ecoregion 16. Recreation, logging, mining, and summer livestock grazing are common land uses; summer livestock grazing also occurs and is more common than in Ecoregion 15.

caption Large areas of rockland and mixed high elevation vegetation occur above extensive Douglas-fir and subalpine fir forests in the rugged Middle Rockies (17).
In Montana, Ecoregion 17 has been divided into 32 level IV ecoregions (17d–17j, 17l, 17m, 17p–17ac, 17ae–17am) that separate areas of significantly different rock type, climate, water quality, and/or terrain. The Absaroka, Madison, and Gallatin ranges are much wetter than other portions of the Middle Rockies (17). High elevations are treeless, typically snow- and ice-covered, and can be lake-studded. Ecoregion 17's carbonate, igneous, and metamorphic rocks strongly influence hydrological characteristics, fish assemblages, and stream quality.
18. Wyoming Basin
The broad, xeric intermontane Wyoming Basin (18) is punctuated by high hills and low mountains and dominated by grasslands and shrublands. The region is somewhat drier than the Northwestern Great Plains (43) and is nearly surrounded by mountains. Livestock grazing takes place throughout the ecoregion even though many areas lack sufficient vegetation to adequately support this activity.

caption Arid, rolling plains, terraces, and alluvial fans compose the Wyoming Basin (18).
In Montana, Ecoregion 18 is undivided and lies in the rainshadow of the Beartooth Plateau. It includes some of the driest places in Montana and unleached, light-colored soils are common. The potential natural vegetation is mostly sagebrush steppe which is distinct from that of the surrounding ecoregions.
41. Canadian Rockies
Ecoregion 41 extends into northern Montana from Alberta and British Columbia. The ecoregion is generally higher and more snow- and ice-covered than the Northern Rockies (15), and portions are strongly influenced by moist maritime air masses. Melting snow and rainfall are abundant at the higher elevations. Some surplus water is stored in glacial deposits, unconsolidated mountain valley fill, and permeable sedimentary rocks. However, areas underlain by crystalline rocks lack sufficient groundwater storage capacity to prevent overland runoff or to develop groundwater supplies; in these places, base flow is meager and high elevation streams generally flow only during rain and snow melt periods. The highest elevations are treeless, glaciated alpine areas. The potential natural vegetation is mostly subalpine fir, Douglas-fir, and Engelmann spruce. Soils are thin or absent on upper mountain slopes but become deeper and more developed below, especially west of the Continental Divide. Recreation, forestry, and mining are common land uses.

caption The Canadian Rockies (41) ecoregion is composed of high, wet mountains. Significant portions are covered by snowfields and glaciers. Glaciated terrain is common and characterized by U-shaped valleys, moraines, cirques, tarns, and outwash features. The Athabaska Glacier, Canada, is shown.
Ecoregion 41 has been broken into five level IV ecoregions (41a through 41e) that are characterized by different types of climate, potential natural vegetation, rock, and/or water quality. Highest elevations are treeless and often snow- or ice- covered. Ecoregions west of the Continental Divide have much more Pacific influence in terms of climate and tree species than do those on the eastern slopes of the Front Range. Portions are underlain by argillite and quartzite; they have substantially different water quality and fish assemblages than other areas underlain by limestone and dolomite. Ecoregion 41 contains Glacier National Park and the Great Bear, Bob Marshall, and Scapegoat Wildernesses. Recreation is the major land use.
42. Northwestern Glaciated Plains
Ecoregion 42 is transitional between the generally more level, moister, more agricultural Northern Glaciated Plains (46) to the east and the typically more irregular and drier Northwestern Great Plains (43) to the south. The southern boundary of the Northwestern Glaciated Plains (42) is near the limit of continental glaciation and its soils are derived from glacial drift. Hummocky moraines locally occur and are characterized by seasonal and semi-permanent ponds and wetlands. Land use is devoted to cattle ranching and farming.

caption Nearly level to rolling terrain underlain by till is typical of the Northwestern Glaciated Plains (42). Extensive farming of mostly wheat and barley occurs in parts of Ecoregion 42 and has replaced the original shortgrass prairie. This photograph was taken east of Shelby, Montana.
In Montana, Ecoregion 42 has been divided into eleven level IV ecoregions (42b, 42d, and 42i through 42q) that separate areas of different terrain, land use, surficial deposits, soils, potential natural vegetation, and/or climate. Ecoregion 42 extends eastward for hundreds of miles from the foothills of the Rocky Mountains. Its eastern till plain is typified by dark brown soils used for crops and range and its western till plain is an important grain farming area; in between, its central till plain has brown, aridic-ustic soils primarily used for grazing. Outwash terraces and end moraines are found locally in Ecoregion 42 and contain many wildlife-rich ponds and wetlands. Groundwater can be taken from the ecoregion’s glacial deposits and bedrock; yield varies by lithologic unit and shale is usually the least productive rock type.
43. Northwestern Great Plains
Ecoregion 43 is largely an unglaciated, semiarid, and rolling plain that is underlain by shale, siltstone, and sandstone. It contains occasional buttes, badlands, ephemeral-intermittent streams, and a few perennial rivers. Low precipitation and high summer evapotranspiration rates restrict groundwater recharge rates. Rangeland is common, but spring wheat and alfalfa farming also occur; agriculture is affected by erratic precipitation and few opportunities for irrigation. Native grasslands persist, especially in areas of steep or broken topography.

caption Large parts of the Northwestern Great Plains (43) ecoregion are dissected by ephemeral and intermittent streams. Ecoregion 43 is primarily used for rangeland and has a potential natural vegetation of grama - wheatgrass - needlegrass. This photograph shows the Willow Creek area north of Fort Peck Lake in Valley County.
In Montana, 17 level IV ecoregions (43a through 43e, 43g, 43k through 43q, and 43s through 43v) have been recognized; they separate areas of distinct terrain, land use, soil, surficial material, climate, and/or potential natural vegetation. Rolling grassland is characteristic of large parts of Ecoregion 43, but wildlife-rich breaks and rough stony buttes occur locally and can be covered in open ponderosa pine forests or savanna. Rangeland is extensive, especially in areas of aridic soils, poor drainage, or dissected terrain; farmland is found locally in Ecoregion 43 such as on the extensive Quaternary terrace deposits of the Judith River Basin. Most soils have a frigid temperature regime; only the southernmost soils are mesic.


  • The full, original version of this entry is located here: That description contains additional maps, as well as information on the physiography, geology, soil, potential natural vegetation, and the land use and land cover of the ecoregion.

  • PRINCIPAL AUTHORS: Alan J. Woods (Dynamac Corporation), James M. Omernik (USEPA), John A. Nesser (USFS), James Shelden (USFS), and Sandra H. Azevedo (OAO Corporation)
  • COLLABORATORS AND CONTRIBUTORS: Robert Bukantis (MDEQ), Chuck Gordon (NRCS), Bill Volk (BLM), Loren Bahls (Flathead Lake Biological Station, University of Montana), Dan Svoboda (USFS), Wease A. Bollman (Rhithron Biological Associates, Missoula, Montana), Thomas R. Loveland (USGS), Anthony Selle (USEPA), Alisa Gallant (Raytheon STX Corporation, Science Applications Branch, EROS Data Center), Cliff Montagne (Land Resources and Environmental Science, Montana State University), John Donahue (Department of Geography, University of Montana), and Jeff Comstock (OAO Corporation)
  • CITING THIS POSTER: Woods, Alan J., Omernik, James, M., Nesser, John A., Shelden, J., Comstock, J.A., Azevedo, Sandra H., 2002, Ecoregions of Montana, 2nd edition (color poster with map, descriptive text, summary tables, and photographs). Map scale 1:1,500,000.
  • This project was partially supported by funds from the Environmental Protection Agency's Office of Research and Development, Regional Applied Research Effort (RARE) program.

Literature Cited

  • Bailey, R.G., Avers, P.E., King, T., and McNab, W.H., eds., 1994, Ecoregions and subregions of the United States (map) (supplementary table of map unit descriptions compiled and edited by McNab, W.H. and Bailey, R.G.): Washington, D.C., USFS, scale 1:7,500,000.
  • Bryce, S.A., Omernik, J.M., and Larsen, D.P., 1999, Ecoregions – a geographic framework to guide risk characterization and ecosystem management: Environmental Practice, v. 1, no. 3, p. 141-155.
  • Chapman, S.S., Bryce, S.A., Omernik, J.M., Despain, D.G., ZumBerge, J., and Conrad, M., 2003, Ecoregions of Wyoming (color poster with map, descriptive text, summary tables, and photographs): Reston, Virginia, U.S. Geological Survey (map scale 1:1,400,000).
  • Commission for Environmental Cooperation Working Group, 1997, Ecological regions of North America – toward a common perspective: Montreal, Commission for Environmental Cooperation, 71 p.
  • Gallant, A.L., Whittier, T.R., Larsen, D.P., Omernik, J.M., and Hughes, R.M., 1989, Regionalization as a tool for managing environmental resources: Corvallis, Oregon, U.S. Environmental Protection Agency, EPA/600/3-89/060, 152 p.
  • Griffith, G.E., Omernik, J.M., Wilton, T.F., and Pierson, S.M., 1994, Ecoregions and subregions of Iowa – a framework for water quality assessment and management: Journal of the Iowa Academy of Science, v. 101, no. 1, p. 5-13.
  • McGrath C.L., Woods A.J., Omernik, J.M., Bryce, S.A., Edmondson, M., Nesser, J.A., Shelden, J., Crawford, R.C., Comstock, J.A., and Plocher, M.D., 2002, Ecoregions of Idaho (color poster with map, descriptive text, summary tables, and photographs): Reston, Virginia, U.S. Geological Survey (map scale 1:1,350,000).
  • Omernik, J.M., 1987, Ecoregions of the conterminous United States (map supplement): Annals of the Association of American Geographers, v. 77, no. 1, p. 118-125, scale 1:7,500,000.
  • Omernik, J.M., 1995, Ecoregions – a framework for environmental management, in Davis, W.S. and Simon, T.P., eds., Biological assessment and criteria-tools for water resource planning and decision making: Boca Raton, Florida, Lewis Publishers, p. 49-62.
  • Omernik, J.M., Chapman, S.S., Lillie, R.A., and Dumke, R.T., 2000, Ecoregions of Wisconsin: Transactions of the Wisconsin Academy of Sciences, Arts, and Letters, v. 88, p. 77-103.
  • U.S. Department of Agriculture–Soil Conservation Service, 1981, Land resource regions and major land resource areas of the United States: Agriculture Handbook 296, 156 p.
  • U.S. Environmental Protection Agency, 2000, Level III ecoregions of the continental United States (revision of Omernik, 1987): Corvallis, Oregon, USEPA – National Health and Environmental Effects Research Laboratory, Map M-1, various scales.
  • Wiken, E., 1986, Terrestrial ecozones of Canada: Ottawa, Environment Canada, Ecological Land Classification Series no. 19, 26 p.
  • Woods, Alan J., Omernik, James, M., Nesser, John A., Shelden, J., and Azevedo, Sandra H., 1999, Ecoregions of Montana (color poster with map, descriptive text, summary tables, and photographs): Reston, Virginia, U.S. Geological Survey (map scale 1:1,500,000).

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Omernik, J., & Griffith, G. (2008). Ecoregions of Montana (EPA). Retrieved from


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