IDAHO, US-BASED UTILITY AVISTA Corporation may face costs as much as US$60M in provisions to address the problem of dissolved gases downstream of its Cabinet Gorge dam on Clark Fork river. The money is to be spent in the period up to 2020. Avista has already submitted a plan to spend US$37M in the first part of the project.

The Cabinet Gorge modifications are part of the licensing conditions the utility agreed to when the Federal Energy Regulatory Commission (FERC) relicenced the hydro power facility several years ago. At Cabinet Gorge, the dam produces dissolved gases downstream in the Clark Fork only during spring runoff, when huge volumes of water are released over the dam’s spillway. The gases are dissolved when water comes into contact with the air as it goes over the spillway at high speeds, and causes a potentially fatal bubble disease similar to bends in fish.

Avista’s proposal for reducing the level of dissolved gases involves modifying two now-closed tunnels at the Cabinet Gorge dam that were created to divert the river during the dam’s construction in the early 1950s. The modifications would allow the tunnels to be opened so that the utility could divert water through them during high-water periods, then close them again when the water level falls.

Reducing the volume of water that goes over the dam’s spillway is expected to lower the level of dissolved gases in the water below the dam. Work on the first tunnel would cost about US$37M and would take place between 2004 and 2009.