Experts are astounded that chocolate brown water from a flash flood 55 miles upstream could flow through two reservoirs to reach the capital city.
The sediment carried to Boise is too fine to hurt fish and wildlife, biologists say. But because fish have trouble seeing lures through the murk, it is temporarily impossible to catch anything on the portion of the Boise River that runs through Ada County.
“It’s something that we didn’t think would happen,” Boise National Forest spokesman Frank Carroll said. “The large fires of 1994 are beginning to affect Boise, in a way we didn’t expect.”
The Aug. 24 floods occurred about 15 miles east of Idaho City, when intense thunderstorms dropped up to 2 inches of water onto parts of the forest scorched by 1994 wildfires.
Rainfall rushed down hillsides, turning quiet creeks into torrents of mud. It shifted the North Fork of the Boise 100 yards west.
Most mysterious is how the brown water managed to pass through the Arrow Rock and Lucky Peak reservoirs without entirely changing the color of the pools. The phenomenon led Forest Service officials to at first discount the murk in Boise was caused by floods 55 miles upstream.
The Army Corps of Engineers and the Bureau of Reclamation, which manage the reservoirs, believe the murky water is entering Arrow Rock Reservoir, sinking to the bottom, and remaining below the surface until it emerges from the outflows of Lucky Peak Dam.
Idaho Fish and Game biologist Scott Grunder predicts bull trout and rainbow trout populations will be devastated in the North Fork for at least two years. He said fish in the Boise area will not be hurt because the sediment is not settling on the river bottom.