COLUMBIA RIVER FISH MITIGATION
Lower Snake River dam breach study
In a seven year effort from 1995 - 2002 the Corps studied four major alternative ways to improve juvenile salmon passage through the hydropower system on the lower Snake River. One of the alternatives studied in the Lower Snake River Juvenile Salmon Migration Feasibility Study was breaching the four dams -- Lower Granite, Little Goose, Lower Monumental and Ice Harbor -- to return a 140 mile stretch of natural river. The other pathways evaluated were: maintain the existing system with planned improvements to fish passage systems; maximize barge/truck transport of juvenile fish around the dams; and, make major system improvements such as surface bypass systems for juvenile fish.
The Corps extensively evaluated the benefits, feasibility and potential impacts of breaching the dams and of the other alternatives. Initial evaluation included seasonal spillway crest level, seasonal natural river level, and permanent natural river level drawdowns of the Snake River dams. Of these, only the permanent natural river option, or dam breaching, was recommended for further examination, based on biological effectiveness, technical feasibility, cost, and other factors. The main obstacles to intermediate level drawdown such as spillway crest level were: 1) the need to make extensive and costly modifications to the fish passage systems at the dams, since fish bypass will continue to be necessary; and 2) evaluations which indicate that salmon survival would not be as high under intermediate drawdown scenarios as under current conditions.
To breach the lower Snake River dams, the earthen portion of each dam would be removed to return the river to a free-flowing state. The concrete-and-steel structures that currently provide power and navigation could remain in place but they would be inoperative. Barge traffic through the lower Snake to Lewiston, Idaho would be discontinued; power production would stop.
The Corps completed the Lower Snake River Juvenile Salmon Migration Feasibility Report/Environmental Impact Statement in September 2002. The study determined that the economic, user and environmental impacts of dam breach would be substantial, without an indication that the action by itself would recover listed stocks of salmon and steelhead. The Corps chose "adaptive migration" (major improvements to fish passage at the dams) as the preferred alternative. The Walla Walla District Web site has more information on the Lower Snake River Juvenile Salmon Migration Feasibility Study.