Large Glaciated Valley Intermountain River
Provisional State Rank
(see reason below)
State Rank Reason
Diversion, flood control, and hydroelectric dams have significantly altered large portions of this Large River Ecosystem in Montana. Threatened White Sturgeon and Bull Trout populations have been affected by Libby Dam operations for the last 40 years.
This ecosystem is found widely throughout the glaciated upper Columbia River basin of the Northern Rockies and Pacific Northwest. Abell et al. (2000) delineated two freshwater ecoregions (FWE) for western Montana, the glaciated (FWE 120) and non-glaciated Columbia (FWE 121). The former marks the glaciated portion, or upper third, of the Columbia River basin (McPhail and Lindsey 1986). This part of the Columbia River basin was glaciated, forming valleys filled with glaciofluvial and morainal sediments; it includes most of eastern Washington, the northern portion of Idaho, the northwestern corner of Montana, and southeastern British Columbia. These major rivers originate on the west side of the Continental Divide and flow westward across areas of uplift toward the Pacific Ocean (McPhail & Lindsey 1986). Main rivers are large tributaries to the Columbia: the Yakima, Okanagan, Spokane, and Kootenai rivers. These large (7th stream order and larger), cold-water rivers have low to moderate gradient, elevations below 1000m, and are characterized by long deep runs and pools with depths >2m, numerous mid-stream islands, side channels and interspaced riffles. Substrate characteristics are typically cobble in the riffles and sand and gravel in the runs and pools, with gravel and/or finer-textured side channels.
These ecological systems are 7th stream order and larger (>200 river miles) cold-water rivers that have a low to moderate gradient with elevations below 1000m. Diagnostic fish species include members of the upper Columbia, Large Valley Intermontane River Community. This assemblage is dominated by large bodied sucker, trout, and minnow species, in order of occurrence frequencies : Largescale Sucker, Longnose Dace, Mountain Whitefish, Longnose Sucker, Redside Shiner, Northern Pikeminnow Bull Trout, Slimy Sculpin, Westslope Cutthroat Trout, Burbot, Mottled Sculpin and the Peamouth. It also includes the Columbia River distributions of the pygmy whitefish (Prosopium coulteri) and white sturgeon (Kootenai River populations). Redband rainbow trout (Oncorynchus mykiss gairdneri), longnose sucker (Catostomus catostomus), burbot (Lota lota), and slimy sculpin (Cottus cognatus) occur primarily in the glaciated portion of the Columbia River basin. Macroinvertebrate Species consist of members of the Medium Cool-Water Transitional Assemblage, the Traditional Trout Stream assemblage, Medium Mountain Stream Community, and the Foothills Transitional Assemblage. The community indicator species are characterized by main channel, fast current stonefly and caddisfly species: Pteronarcys californica, Hesperoperla pacifica, Brachycentrus americanus, Arctopsyche grandis, Hydropsyche, Glossosoma, Lepidostoma and the tipulids: Hexatoma and Antocha . Mayflies are diverse in this system and contain many genera, including: Baetis, Ephemerella, Serratella, Rhithrogena, Drunella and Epeorus . As these transitional rivers proceed downstream and begin to warm (>17 C) or are sediment-impaired, degraded, or dewatered, they will quickly lose the Traditional Trout Stream and Medium Mountain Stream Community (#4 and 90), and shift to the mayfly, caddisfly, beetle and dipteran species that form the Medium Cool-Water Transitional Assemblage (#1) and the Foothills Transitional Assemblage (#105), with indicator species: Hydropsyche, Optioservus, Baetis tricaudatus, Brachycentrus occidentalis, Helicopsyche borealis, Prosimulium, Amiocentrus aspilis, Lara, Plauditus, Narpus. Populations of the western pearlshell mussel have been reported from this river ecosystem, although the populations may be in decline due to loss of their native host fish the Westslope Cutthroat Trout.
The Large Intermountain Glaciated Valley River type occurs within the Kootenai River section in Montana from below Libby Dam to the Idaho state line continuing downstream towards Bonners Ferry. This Aquatic Ecological System is found more widespread in other rivers of British Columbia.
Large dams and reservoirs have had the most significant negative impact on this ecological community. Dams create barriers to the long distance spawning runs many fish in this community need, and reservoirs have submerged considerable spawning habitat. Inter-dam reaches below Libby and above Bonner's Ferry maintain some of their pre-development channel morphology, but they are affected by altered water temperatures, unnatural water level fluctuations, and changes in sediment and nutrient transport.