Work could boost river's health

SANDPOINT — Recent improvements to the Cabinet Gorge and Noxon Rapids dams are expected to result in a healthier Clark Fork River.

With construction on the enhancements completed in January, Avista Utilities is currently performing tests on the additions of ski-jump style spillways and “bumps,” also known as baffle blocks. The thorough round of tests will prepare the dams for the peak flows that take place during spring runoffs.

Avista engineers aim to use these prefabricated concrete blocks and redesigned spillways to reduce total dissolved gas, or TDG. That’s good news for the region’s ecology, because TDG can be harmful to the fish and aquatic insects that populate the Clark Fork River.

According to Avista engineer Guy Paul, the design is not only good for the environment — it’s also raising eyebrows from an engineering standpoint.

“Putting baffle blocks on a spillway has been done for many years, primarily to reduce erosion at discharge points,” Paul said. “To our knowledge, this is the first time the concept has been implemented solely to reduce TDG.”

Paul describes the improvement as being similar to a twisting spray nozzle used for garden hoses, which can be tightened for a thin, pressurized stream of water or a more widely-dispersed spray. By building the spillway with these improvements in mind, Avista engineers are able to more broadly disperse the energy of water flows.

“Doing this on the spillway means less air is sucked into the river water,” Paul said. “It’s not taken to great depth. Water becomes less supersaturated.”

Reduced TDG levels are important for local wildlife, and federal and state standards are in place to make sure the ecological balance isn’t disrupted. Too much TDG can inflict gas bubble trauma on river fish. This condition occurs when too much nitrogen in the animal’s bloodstream causes lesions to form in its body tissue. If these lesions become infected, the fish may sicken or die.

Testing is still under way for the improvements, but so far, engineers are reporting positive preliminary results. Based on their final report, Avista officials may order the improvements for additional dam bays. According to officials, other hydroelectric facility officials across the Pacific Northwest are also keeping an eye on the progress with keen interest as they attempt to reduce their own TDG levels.