Montana Field Guide

Prairie potholes occur in shallow depressions scraped out by glaciers in the northern Great Plains of Montana. The region is characterized by a glacial landscape of end moraines, stagnation moraines, outwash plains and lake plains. The glacial drift forms steep to slight local relief with fine-grained, silty to clayey soils. Limestone, sandstone, and shales are the predominant parent materials, and highly mineralized water can discharge from these rocks. The hydrology of this system is complex, and the concentration of dissolved solids results in water that ranges from fresh to extremely saline, with chemical characteristics varying seasonally and annually. Most prairie potholes and associated lakes contain alkaline water, which accumulates rapidly in during spring months, especially when soil frost is sufficiently deep to forestall all infiltration until after the ground thaws. Most water loss occurs through evapotranspiration, which exceeds precipitation during summer months. Vegetation within this system is highly influenced by hydrology, salinity and dynamics. Potholes can vary in depth and duration, which determines the local gradient of plant species. Similarly, species found within individual potholes will be strongly influenced by periodic drought and wet periods. The wettest sites, where water stands through summer, are characterized by hardstem bulrush (Schoenoplectus acutus), often occurring as a near monoculture, or with softstem bulrush (Schoenoplectus tabernaemontani) or common threesquare (Schoenoplectus pungens) along slightly drier margins. In permanently flooded sites, aquatic buttercups (Ranunculus species), aquatic smartweeds (Polygonum species), pondweeds (Potamogeton species) or duckweeds (Lemna species) are common. At the drier extremes, pothole vegetation generally occurs in a concentric pattern from a wetter middle dominated by spikerush (Eleocharis species) through a drier ring of foxtail barley (Hordeum jubatum) and an outer margin of western wheatgrass (Pascopyrum smithii) or thickspike wheatgrass (Elymus lanceolatus). Prairie potholes are considered to be the most important breeding habitat for waterfowl in North America, with production estimates ranging from 50% to 80% of the continent’s main species. However, the extreme variability in climate and pothole water levels also results in extreme fluctuations in waterfowl populations from year to year. Prairie pothole wetlands also support a diverse assemblage of water-dependent birds.

In Montana, most prairie potholes are concentrated north of the HiLine and Route 2, from the Blackfeet Reservation to the North Dakota border, although individual potholes occur across the Northwest Glaciated Plains north of the Missouri River. Elsewhere, this system occurs throughout the northern Great Plains from central Iowa northeast to southern Saskatchewan and Alberta. It encompasses approximately 870,000 square kilometers with approximately 80% of its range in southern Canada. It is also prevalent in North Dakota, South Dakota, and northern Minnesota.

The prairie pothole ecological system is dominated by closed basins that receive irregular inputs of water from the surroundings and export water as groundwater. The climate is characterized by mid-continental temperature and precipitation extremes. The region is distinguished by a thin mantle of glacial drift with overlying stratified sedimentary rocks of the Mesozoic and Cenozoic ages; these form a glacial landscape of end moraines, stagnation moraines, outwash plains and lake plains. The glacial drift is from 30 meters to 120 meters thick and forms steep to slight local relief with fine-grained, silty to clayey soils. Limestone, sandstone, and shales are predominant, and highly mineralized water can discharge from these rocks. Precipitation and runoff from snowmelt are often the principal water sources, with groundwater inflow as a secondary source. Evapotranspiration is the primary source of water loss, with seepage loss secondary. The hydrology of this system is complex, and the concentration of dissolved solids results in water that ranges from fresh to extremely saline, with chemical characteristics varying seasonally and annually. Most prairie potholes and associated lakes contain water that is alkaline (pH >7.4). Surrounding uplands are generally in cropland (small grains), hay, or range.

Prairie potholes are considered to be the most important breeding habitat for waterfowl in North America, with production estimates ranging from 50% to 80% of the continent’s main species. However, the extreme variability in climate and pothole water levels also results in extreme fluctuations in waterfowl populations from year to year. Prairie wetlands also support a diverse assemblage of water-dependent birds including Montana species of concern such as the black-crowned night heron (Nycticorax nycticorax), white-faced ibis (Plegadis chihi), Franklin’s gull (Larus pipixcan), common tern (Sterna hirundo), Forster’s tern (Sterna forsteri), and black tern (Chlidonias niger). American white pelicans (Pelecanus erythrorhynchos) feed extensively on tiger salamanders (Ambystoma tigrinum) found in prairie potholes. Sparsely-vegetated alkali potholes, especially in Sheridan County, are attractive to piping plovers (Charadrius melodus).

Vegetation within this system is highly influenced by hydrology, salinity and dynamics. This system includes elements of emergent marshes and wet, sedge meadows that develop into a pattern of concentric rings. Potholes can vary in depth and duration, which determines the local gradient of species. Similarly, plant species found within individual potholes will be strongly influenced by periodic drought and wet periods. The wettest sites, where water stands into or through summer, are characterized by hardstem bulrush (Schoenoplectus acutus), often occurring as a near monoculture, or with a fringe of softstem bulrush (Schoenoplectus tabernaemontani) or common threesquare (Schoenoplectus pungens) along slightly drier margins. Cattails (Typha spp) are also seen in these wetter systems, although they are typically a minor component. During spring or in permanently flooded sites, aquatic buttercups (Ranunculus species), aquatic smartweeds (Polygonum species), pondweeds (Potamogeton species) or duckweeds (Lemna species) may be abundant. At the drier extremes, pothole vegetation generally occurs in a concentric pattern from a wetter middle dominated by spikerush (Eleocharisspecies) through a drier ring of foxtail barley (Hordeum jubatum) and an outer margin of western wheatgrass (Pascopyrum smithii) or thickspike wheatgrass (Elymus lanceolatus) (Hansen et al, 1996; Lesica 1989). Grazing, draining, and mowing of this system can influence vegetation distribution.

Livestock use of potholes is limited by low palatability of characteristic species, although open water attracts livestock for both drinking and cooling. When upland vegetation becomes sparse, cattle will graze on spikerush and bulrush. Wet soils are easily trampled. Grazing, when properly planned and executed, can be a management tool, preventing cattail encroachment into open water, limiting the spread of exotics such as crested wheat (Agropyron cristatum) and smooth brome (Bromus inermis), and avoiding excessive litter buildup. Prescribed burning can be used to the same ends. Prairie potholes are primarily threatened by crop agriculture, by unrestricted grazing, and by oil and gas development. Region-wide, nearly half of this system has been lost.

In Great Plains prairie pothole wetland systems where water has been drained or altered, 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. Many of the characteristic species found in marsh systems are rhizomatous, thus exhibit excellent erosion control properties. However, species that are infrequent in these wetland systems may not re-occur or re-establish in a given time frame. The major barriers to prairie pothole restoration are isolation, infrequent flooding and invasive species. These factors must be addressed during the planning and long term management of restored prairie pothole wetlands.

During restoration, cattle grazing needs to eliminated or controlled to allow regrowth, recolonization and resprouting from existing root systems. In some cases, if hydric soils are heavily altered due to pugging or compaction, addition of organic material may be needed to facilitate plant recolonization.