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Montana Field Guides

Transitional Foothills and Valley Rivers

Provisional State Rank: S4
(see reason below)

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State Rank Reason
Although widespread and with many representative river reaches in the state, this ecosystem is negatively affected by small dams, water diversions, stock ponds and introduced gamefish species. Therefore, it is fairly rare to find biologically intact river miles of this ecological system.

General Description
This ecosystem is found in the moderate elevation (1200-2000m), upland foothill and valley streams of the Middle Rockies, including the isolated mountain ranges. The smallest sub-type, the Headwaters Foothills River, is a 1st or 2nd order stream tributary to the Small Transitional and Small Foothills River Systems which are small-to-medium sized (2nd-4th order, average wetted width=5m), moderately flowing rivers with permanent flow and strong seasonal variability from melting snowpack. These streams represent the transition from high gradient mountain stream communities to the intermontane or prairie rivers in the east. Small Foothills Rivers are tributaries to Intermountain Rivers and maintain average summer temperatures (<20 degrees C) suitable for cool- to coldwater- fish species. In contrast, Small Transitional Foothill Rivers typically enter Prairie Rivers to the eastern part of the state and lose the ability to maintain cool-water fish species at the lower elevations. The surrounding landscape is transitional as well, with coniferous forests giving way to shrubland and sage steppe. Willows and broad-leaf trees dominate riparian areas. Beaver pond complexes are often found on these streams. Beavers find suitable conditions and dam building materials as the foothills stream gradient decreases. These ponds provide substantial trout habitat, but can warm beyond the tolerance of cutthroat trout, and the native cutthroats will usually be pushed to the upstream reaches by the more aggressive brook trout. The substrate of these streams is usually boulder/cobble riffles, gravel/sand runs and pools, and silted-in beaver ponds with large woody debris.

Diagnostic Characteristics
The fish community is the Coolwater Transitional Community and the Traditional Trout Stream Assemblage with indicator species of the Headwaters Foothills and Small Foothills Rivers characterized by native species, the Westslope or Yellowstone Cutthroat trout, mountain whitefish and mottled sculpin. However, the introduced species of the Stocked Trout Assemblage, the brook trout and rainbow trout, tend to dominate and become the focal species of these systems. As Small Foothills and Transitional Foothills Rivers proceed down gradient, inclusions of the longnose sucker, longnose dace and the mountain sucker (Yellowstone drainages) into the community becomes apparent. Additional fish species (the exotic carp, white sucker, lake chub and fathead minnows of the Core Prairie Stream Assemblage) may be found at the warmer, lower end of the Small Transitional Foothills Rivers in the prairie regions. The shallow riffle areas of the larger foothills rivers entering Intermountain Rivers may provide spawning habitat for downstream populations of brown trout during their fall migration, and suckers, dace and rainbow trout in the spring. Macroinvertebrate Community: Headwaters and Small Foothills Rivers are dominated by the Traditional Trout Stream Assemblage, with some members of the Medium Coolwater Transitional and Foothills Transitional Assemblages. The community indicator species are characterized by main channel, fast current mayfly, stonefly and caddisfly species (Pteronarcys californica, Hesperoperla pacifica, Brachycentrus americanus, Rhithrogena, Arctopsyche grandis, and Lepidostoma spp.), and the tipulid, Antocha. As Small Foothills Rivers proceed downstream and begin to warm (>17 °C) or are sediment impaired, degraded or dewatered, they will quickly lose the Traditional Trout Stream Assemblage and shift to the mayfly, caddisfly, beetle and dipteran species that form the Medium Coolwater Transitional Assemblage with indicator species Hydropsyche, Optioservus, Baetis tricaudatus, Brachycentrus occidentalis, Helicopsyche borealis, Corynoneura, Constempellina, Prosimulium, Amiocentrus aspilis, Lara, Plauditus, and Narpus. Lower, warmer stretches of Small Transitional Foothills Rivers will begin to pick up species of the Transitional Prairie River Assemblage. Populations of the western pearlshell mussel have been documented in the Foothills Transitional river ecosystem, although these populations may be in decline.

This C001 stream type has been identified in the Lewis and Clark National Forest foothills of the Belt, Judith and other isolated mountain ranges. The Middle Missouri, Sun, Teton, Musselshell drainages contain the most representatives of this community type in the database including Arrow, Dog, Newlan Creek, Elk Creek, and Sixteenmile Creek, while the C001y AES sub-class is found within the Yellowstone Foothills.

Spatial Pattern

Dynamic Processes
Beaver played a large role in the ecological processes of this ecological system in the past and provided mediating flood control with their numerous beaver ponds in the watershed. Large riparian willow complexes were indicative of a proper functioning small mountain to foothill transitional stream.

Livestock use around the riparian areas of this ecosystem is common and can have strong local effects resulting in sedimentation, a shift of the macroinvertebrate communities from a Traditional Trout Stream Assemblage to the Medium Coolwater Transitional Assemblage, and a shift in fish communities from native cutthroat trout to introduced species, such as brook trout. High-density cattle usage can cause severe degradation, bank erosion, sedimentation and siltation on the riffle habitats and gravel spawning areas downstream. Water diversions lower in the foothills for agriculture may be an issue since these streams usually flow onto private lands as foothills grade into the valleys and decrease in elevation and gradient.

  • Classification and Map Identifiers

    1310: Transistional Foothills and Valley Rivers

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Citation for data on this website:
Transitional Foothills and Valley Rivers — Transistional Foothills and Valley Rivers.  Montana Field Guide.  Retrieved on , from