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Impact of Libby dam on River floodplain physical processes and ecosystem at the Kootenai River, ID, USA

Funding Agents: Bonneville Power Administration, Fish and Wildlife Mitigation Program, Kootenai Tribe of Idaho
PIs: Dr. Klaus Jorde and Dr. Elowyn Yager
Students: Berhon Dibrani, Karen Zelch, Michael Burke, Rohan Benjankar and Yi Xie
Contacts: Klaus Jorde and Elowyn Yager, Center for Ecohydraulics Research, University of Idaho, Boise, ID.

Project Objectives:

• Develop modeling tools (1D and 2D hydrodynamic models) that quantify the impacts of Libby Dam and levees on physical processes (flow, sediment transport, channel morphology changes etc.) that act in the river and on the floodplain. The impacts are based on a comparison of current conditions and reference scenarios before the dam and levee construction.
• Develop models that calculate the feedback between physical and biological processes (e.g. vegetation establishment and recruitment) to determine human impacts on river and floodplain ecology.
• Use hydraulic, sediment transport and vegetation models to evaluate future restoration and mitigation locations and techniques in the Kootenai River and floodplain.

The Kootenai River Basin is an international watershed 36,000 km² in size, encompassing parts of British Columbia, Montana, and Idaho. The headwaters are located in Kootenay National Park, British Columbia. It is the second largest tributary to the Columbia in terms of runoff volume, and the third largest in terms of watershed area.

The construction of Libby Dam in 1972 significantly altered the hydrograph, fish migration patterns, temperatures, nutrient availability, sediment transport, and vegetation dynamics below the dam. Many of the tributaries in the canyon portion of the Kootenai River have experienced interstitial flow in the summer due to the development of large alluvial fans and low water levels.

Regulation of the Kootenai River between Libby Dam and Kootenay Lake has altered the natural flow regime, resulting in a significant decrease in maximum flows (60% net reduction in median 1-day annual maximum, and 77-84% net reductions in median monthly flows for the historic peak flow months of May and June, respectively). Bed shear stress and average velocity in the channel showed the largest change, which was predicted to be 79 and 55% of pre-regulation conditions, respectively.

The Kootenai River floodplain is almost completely disconnected from its main channel because of levees and the altered hydrological regime from Libby Dam. The decrease in flood frequencies and magnitudes combined with extensive river modification has completely changed the physical processes and vegetation dynamics that act on the floodplain. Our floodplain simulation showed that more than 90% of previously inundated areas have been lost due to river modification (dyke and levees) and dam operation.