The Inter-Mountain Basins Playa ecological system describes barren or sparsely vegetated playas (generally <10% plant cover) throughout the Intermountain West. These systems are intermittently flooded and water is typically prevented from percolating through the soil by an impermeable soil subhorizon and is left to evaporate. Some are affected by high groundwater tables. Soil salinity varies with soil moisture and greatly affects species composition. Salt crusts are common throughout, with small saltgrass beds in depressions and sparse shrubs around the margins. Characteristic species in Colorado may include black greasewood (Sarcobatus vermiculatus), spiny hopsage (Grayia spinosa), Lemmon's alkali grass (Puccinellia lemmonii), Great Basin wildrye (Leymus cinereus), inland saltgrass (Distichlis spicata), and species of saltbush (Atriplex spp.). These wetlands are particularly important to waterfowl and shorebird and also support many rare and unique species.

This system describes playas throughout the Intermountain West with saline soils that are intermittently flooded or are maintained by a perennially high water table. They support a sparse cover (generally < 10%) of salt-tolerant vegetation that includes both shrubs and herbs.

This system occurs throughout the Intermountain West. Colorado occurrences are primarily in the San Luis Valley.

Inter-mountain Basins Playas are a small patch system.

The landscape of the Intermountain West is characterized by north-south trending mountains, with large alluvial fans at their base, separated by broad, sediment-filled valleys (basins), many of which have internal drainages. Mountain-building uplift and volcanism events with subsequent erosion and depositional processes created the inter-mountain basins that are now filled with deep alluvial deposits derived from erosional processes in the nearby mountains. Playas and similar saline wetlands occur in the inter-mountain basins, where the landscape provides the hydrogeomorphic template that enables their development. Playas are often described as closed basin systems whose hydrological inputs are limited to precipitation and surface runoff. On the surface, this definition makes sense for playas in the Intermountain West. For example, in the Closed Basin of the San Luis Valley, Colorado, playa systems form at the base of terminal streams flowing from nearby mountain ranges. However, many of the playas found in the Intermountain West, including those in the Closed Basin in the San Luis Valley, are also subject to groundwater discharge or capillary movement of water from seasonally high water tables. As a result, these wetlands are characterized by complex interactions of surface and ground water. Regardless of their hydrological source, Inter-mountain Basin Playas share similar soil chemistry as well as floristics with many stereotypical, precipitation-fed playas such as those found in the Southern High Plains of Texas and New Mexico.

Climate within Colorado's inter-mountain basins is strongly influenced by the mountains, but also semi-arid with warm, dry summers and cold winters. Most precipitation occurs as snowfall (as much as 80% at high elevations) during the winter months and thus is the most important source of water for wetlands and riparian areas in the Southern Rocky Mountains. However, late-summer convective thunderstorms produce slight peaks in runoff in late summer. Evaporation generally exceeds precipitation, especially at lower elevations and in the inter-mountain basins. The ratio between evaporation and precipitation has a strong influence on the soils and hydrology of wetlands throughout the region.

Interaction of climate and geomorphology has a strong influence on local wetland hydrological processes. In Colorado, snowmelt at high elevations contributes a large proportion of water to most wetland types through its influence on groundwater and surface water dynamics. Inter-mountain basins receive surface water from snowmelt streams originating in the surrounding mountains. These streams can terminate in depressions or basins that have no drainage outlet and have impermeable soils, conditions that set the stage for playa formation. Many playas fill from snowmelt-fed streams in late spring but most are dry by late summer, although heavy monsoon precipitation can cause some plays to refill in late summer and some may wet up only during high precipitation years. Most water loss occurs by evaporation, as vegetation cover is sparse and losses to groundwater are low due to the characteristic alkali clay soils that prevent infiltration. High rates of evaporation results in a high concentration of evaporative salts in the upper soil profile, conditions which select for flora and fauna that are adapted to seasonal soil saturation and saline conditions.

Some playas are maintained by groundwater, with groundwater recharge ultimately dependent on subsurface or surface flows from adjacent mountains. For example, many of the playas in the San Luis Valley, Colorado, depend upon a complex interaction of surface and groundwater sources that undergo characteristic seasonal and inter-annual fluctuations. In these playas, when the groundwater table approaches the soil surface, capillary action can bring salts and water up to or near the soil surface. When this capillary water reaches the soil surface, high evaporation rates leave increased concentrations of salts in the upper soil horizons and on the soil surface as a salt crust. Salt concentrations in these wetlands can be up to 500 times that found in freshwater wetlands.

Species composition is strongly influenced by water and soil salinity. Thus playas exhibit distinct bands or zones of vegetation that vary according to soil salinity and water table levels or duration of inundation. Regularly flooded playas support well developed aquatic and shoreline emergent vegetation, such as pondweeds (Potamogeton spp.), horned pondweed (Zannichellia palustris), spikerush (Eleocharis acicularis), and hardstem bulrush (Schoenoplectus acutus). Sites that are inundated for short durations (1 to 3 months) are characterized by emergent species, such as three-square bulrush (Schoenoplectus pungens) and common spikerush (Eleocharis palustris). Areas of seasonally high water tables and saline soils are characterized by shallow emergent species such as Nevada bulrush (Amphiscirpus nevadensis) and mountain rush (Juncus balticus var. montanus). Salt flats, where capillary action results in an abundance of salt crusts on the soil, are tolerated by only a few plant species such as western glasswort (Salicornia rubra) and alkali bulrush (Schoenoplectus maritimus). Seasonally saturated saline wet meadows are characterized by species such as inland saltgrass (Distichlis spicata), three-square bulrush (Schoenoplectus pungens), and Baltic rush (Juncus balticus). Mesic soils with various salinity levels support inland saltgrass (Distichlis spicata) while adjacent alkali flats and dunes dominated by greasewood (Sarcobatus vermiculatus) and rabbitbrush (Chrysothamnus spp.) respectively. Other plant species commonly found in playa systems include alkali sacaton (Sporobolus airoides), saltbush (Atriplex spp.), scratchgrass muhly (Muhlenbergia asperifolia) and saline plaintain (Plantago eriopoda).

Playa wetlands support many rare plant and animal species. Playas of the San Luis Valley support large populations of the globally rare slender spiderflower (Cleome multicaulis), the Great Sand Dunes tiger beetle (Cicindela theatina), the San Luis sandhill skipper (Polites sabuleti ministigma) and two mammal subspecies, the plains pocket mouse (Perognathus flavescens relictus) and silky pocket mouse (Perognathus flavus sanluisi).

Playa wetlands are particularly important waterfowl and shorebird habitats. Variability in water levels and salinity and the subsequent vegetation types support a variety of aquatic and terrestrial invertebrates that then provide foraging resources for a variety of bird species. Six state rare bird species have been documented at the playas in Great Sand Dunes including Short-eared Owl (Asio flammeus), Snowy Plover (Charadrius nivosus), Long-billed Curlew (Numenius americanus), Black-crowned Night-heron (Nycticorax nycticorax), White-faced Ibis (Plegadis chihi), and Forster's Tern (Sterna forsteri). Other bird species that have been documented using playa wetland habitats include Mallard (Anas platyrhynchos), Green-winged Teal (Anas crecca), Cinnamon Teal (Spatula cyanoptera), American Coot (Fulica americana), Blue-winged Teal (Spatula discors), Killdeer (Charadrius vociferous), Common Snipe (Gallinago gallinago), Sora (Porzana carolina), Spotted Sandpiper (Actitis macularius), Wilson's Phalarope (Phalaropus tricolor), American Avocet (Recurvirostra americana), Virginia Rail (Rallus limicola), Red-winged Blackbird (Agelaius phoeniceus), Yellow-headed Blackbird (Xanthocephalus xanthocephalus), Common Yellowthroat (Geothlypis trichas), Song Sparrow (Melospiza melodia), Brewer's Blackbird (Euphagus cyanocephalus), Savannah Sparrow (Passerculus sandwichensis), and Northern Harrier (Circus cyaneus).

The role of disturbance processes for this system is largely unknown. Disturbance may include periodic high water periods, which can drown plants, as well as multi-year droughts, leading to high plant mortality. As wetlands dry out, salt concentrations can increase due to evaporation, eventually exceeding tolerance of some plant species, which may lead to community succession.

Disturbance from some native animal species is important to the survivability of some plant and animal species. Slender spiderflower (C. multicaulis) is restricted to very specific microhabitats of moist alkaline soils. The playas in the San Luis Valley provide these conditions. The slender spiderflower also appears to respond well to some forms of soil disturbance. These discriminating habitat requirements limit the slender spiderflower to the edges of alkaline playa lakes and wetlands. Digging by the Northern Pocket Gopher (Thomomys talpoides macrotis) provides the soil disturbance necessary for reproductive success of the slender spiderflower and also provides the burrows for Burrowing Owls, which occur at the San Luis Valley playa sites.

Playa wetland ecosystems are intimately tied to specific runoff patterns and the water table which undergo characteristic seasonal and inter-annual fluctuations. Anthropogenic changes to the depth or duration of inundation can have profound effects on soil salinity and consequently wetland vegetation and by extension on wetland dependent fauna including waterbirds, amphibians, insects and mammals. Reservoirs, water diversions, groundwater withdrawal, ditches, roads, and human land uses in the contributing watershed which perturb the timing or magnitude of surface- and/or groundwater flows, are likely to affect playas detrimentally.

Non-native species can displace native species, alter hydrology, alter structure, and affect food web dynamics by changing the quantity, type, and accessibility to food for fauna. Numerous invasive and/or exotic species are known to occur in playas. Non-native species that are especially problematic in playa wetlands include common reed (Phragmites australis), whitetop (Cardaria spp.), goosefoot (Chenopodium glaucum and C. rubra), Canada thistle (Cirsium arvense), Russian thistle (Salsola spp.), ironweed (Bassia hyssopifolia), and kochia (Kochia scoparia).

Climate has a pivotal role in the maintenance of playas. Because playa wetland systems are dependent on both surface and groundwater, diminished surface flows and groundwater depletion will likely impact system integrity. Surface flow and groundwater recharge are projected to likely be impacted by global climate change which is altering the timing and character of hydrological processes that maintain wetland systems. Human-induced warming is resulting in the loss of snowpack and earlier snowmelt which is projected to worsen, with trends indicating fundamental impacts on streamflow and water supplies across the western United States. The Southwest is projected to experience longer and more severe droughts from the combination of increased evaporation and reduced precipitation which is projected to impact both surface and groundwater.