Montana Field Guide

This ecosystem is found in the moderate elevation (1200-2000m), upland foothill streams of the Northern Rockies, Canadian Rockies or Middle Rockies Ecoregions. These moderately confined-channel streams are usually small- to medium-sized (2nd-3rd order, average wetted width=5m), with average summer temperature <15°C). Moderately flowing streams have permanent flow with strong seasonal variability due to melting snowpack from higher elevation mountainous areas. They are often within BLM or National Forest Service boundaries. These streams represent the transitional areas from the headwater or forested stream communities to lower foothills and intermontane rivers, and provide substantial habitat for Montana’s native cutthroat trout populations, which thrive in the cold water temperatures and complex in-stream habitats. The geomorphology of these streams is usually a riffle/run/pool configuration with substrate dominated by boulders and cobbles, and gravel in the short pools. Large woody debris from the surrounding hillslopes can provide significant channel material and additional substrate to these streams. The (L) descriptor on this AES code depicts proximity to a lake outlet or large beaver complex which can have significant effect on the biological community.

The fish community is the Coldwater Transitional Community and the Traditional Trout Stream Assemblage with indicator species of the Intermountian Foothill Stream and Rivers characterized by the native species, Westslope Cutthroat trout, mountain whitefish and Rocky Mountain sculpin, with corridor use by the Bull Trout. 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 Intermountain Foothills Streams and Rivers proceed down gradient, inclusions of the bull trout, longnose sucker, and large scale sucker into the community become apparent if stream quality and connectiveness remain intact. Intermountain Streams and Rivers provide spawning habitat for downstream populations of bull trout during their summer/fall migration and cutthroat trout and suckers in the spring. Macroinvertebrate Community of the Intermountain foothills rivers are dominated by the Traditional Trout Stream Assemblage, with some members of the Medium Mountain Stream Assemblages. The community indicator species are characterized by main channel, fast current mayfly, stonefly and caddisfly species (Caudatella, Epeorus, Drunella spp., Rhithrogena, Pteronarcys californica, Classenia sabulosa, Hesperoperla pacifica, Perlodids, Brachycentrus americanus, Arctopsyche grandis, and Lepidostoma spp.), and the tipulid, Antocha. As 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, stonefly and caddisfly species that form the Medium Montain Assemblage, with its indicator species Hydropsyche, Optioservus, Baetis tricaudatus, Brachycentrus occidentalis, Helicopsyche borealis, Corynoneura, Constempellina, Prosimulium, Amiocentrus aspilis, Lara, Plauditus, and Narpus. Populations of the western pearlshell mussel have been documented in the Intermountain Foothills Transitional river ecosystem, although these populations may be in decline when they lose their cutthroat trout host fish.

This AES within the westside river drainages of the state contains 2718.4 miles of stream. C007 classified streams encompass low to moderate gradient intermountain streams from the Glaciated Columbia Region of the Northern Rockies to the Idaho Batholith in the Bitterroot Basin. Representative streams include the Little Blackfoot River, West Fork Rock Creek, Ninemile Creek, the upper Swan River and Thompson Rivers. C009 AES is the Intermontane Moderate Gradient Stream across the Bitterroot Valley Ecological Section. C010 AES is the Intermontane Moderate Gradient Stream across the Flathead Valley Ecological Section. C011 is the Intermontane Moderate Gradient Streams across the glaciated Northern Rockies Valley Ecological Section.

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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.

Small dams, water diversions, stock ponds and introduced gamefish species have had the most significant negative impact on this community (Winston et al. 1991). Other threats include heavy cattle intrusions to the riparian areas, which causes bank erosion and subsequent sedimentation and siltation. Anywhere dams occur, even small stock pond earthen dams, the downstream reaches are affected by altered water temperatures, unnatural water level fluctuations, and changes in sediment and nutrient transport.