If a heat wave hits Southern Idaho, the Boise River won’t be the only river to spill over its banks and cause potentially serious flooding downstream.
Snowpack remains high in much of the state, and the Big Wood River watershed has a record snowpack that is sending unprecedented flows through the Wood River Valley.
In the Treasure Valley, federal water managers plan to raise Boise River flows again this week with the completion of a levee at the Sunroc gravel pit near the head of Eagle Island. Dam managers plan to increase the flows, measured at the Glenwood Avenue bridge, from 8,100 cfs to 8,600 cfs by Wednesday.
“We’re expecting completion of the levee today,” said Kate McGwire, a spokeswoman for Ada County, said Monday.
Federal officials said residents should expect more increases in Boise River flows, because water coming into the Boise River reservoir system remains above 11,000 cfs.
More than 2 million acre-feet of water remains in the Boise watershed in the form of snow, with less than 300,000 acre-feet of capacity left in the Boise River reservoirs. The U.S. Weather Service predicts that when the river rises to 8,900 cfs, severe bank erosion and flooding will occur in low areas close to the river in Boise, Garden City, Eagle and Caldwell, but the water should remain below the level of most residential areas.
The wild cards are trees, which eroding river banks could drop into the river. If a big uprooted tree gets stuck on a bridge, it could create a dam that sends flows flooding in unexpected directions, officials say.
Much of the Boise River Greenbelt is under water already, and officials say increased flows could send water over roads like ParkCenter Boulevard or Eagle Road. Some Idaho water experts say privately that the river could have to be raised to as much as 12,000 cfs for a month to prevent a week of flows as high as 20,000 cfs.
In 1943, before Lucky Peak was built in 1955, 25,000 cfs of floodwater ran through Boise.
AROUND THE STATE
Snowpack remains high across Southern Idaho and in some areas is setting records. Some basins, like the Wood River that runs through Ketchum and Hailey, have little or no upstream storage reservoirs. Others, like the Big Lost River above Mackay, have a snowpack as high as seven times the capacity of its Mackay Reservoir, prompting state officials to urge the irrigation district there to increase releases from the dam.
The Big Wood snowpack is at 180 percent of average, the highest since records began in 1961. Flows on the Big Wood river were running at 1,200 cfs Monday, the highest ever measured for the date in the 101 years the stream gauge has been in place, according to the U.S. Geological Survey.
The Big Lost River is at 195 percent of average, with more than 270,000 acre-feet of water as snow in the watershed; the 75,000 acre-foot Mackay Reservoir already is about half full.
The Big Lost River Irrigation District increased flows in March after Idaho Water Resources Director Gary Spackman warned that his agency fears the reservoir could fill and the district would not be able to prevent “the emergency overflow spillway from discharging uncontrollably to pass all of this expected runoff.”
Spackman wrote a similar letter to the Oakley Canal Co., which operates Goose Creek Reservoir. He said without more releases, the canal company is increasing the risk it could overtop its spillway and cause downstream flooding.
The Payette snowpack is at 132 percent of average, according to the Bureau of Reclamation, far less than the 150 percent of average sitting in the Boise River watershed. But a warm spell could speed the runoff and cause flooding, even with Cascade and Deadwood reservoirs at 75 percent of capacity.
Other watersheds and their snowpacks according to the U.S. Department of Agriculture’s Natural Resources Conservation Service: the Salmon River basin at 130 percent of median; Weiser, mostly melted, at 105 percent; Henrys Fork at 114 percent; the Snake above Palisades Reservoir at 141 percent; and Bear River at 143 percent.