The Boise Basin snowpack grew over the past month as mountain storms delivered 300 percent of normal February precipitation, forcing federal dam managers on the Boise River to prepare to raise the river above flood level with at least four months of runoff still ahead.
That means Treasure Valley residents should prepare for a long flooding season and consider buying flood insurance in low-lying areas. The basin’s snowpack was measured at 140 percent of average March 1, with the streamflow forecasts ranging from 150 percent to 190 percent of normal this year, reported the Idaho Natural Resources Conservation Service.
The U.S. Army Corps of Engineers reported Tuesday that 2.2 million acre-feet of water sits in the three Boise reservoirs and as snow in the mountains of the 2,680 square-mile watershed. That is despite increased flows below Lucky Peak since early February. The releases bought dam managers some capacity, with the three reservoirs at 52 percent, down from 61 percent a month ago.
Water managers from the Corps and the Bureau of Reclamation plan to raise the river to about 7,500 cubic feet per second by Friday. Flood stage is considered to be 7,000 cfs at the river gauge at the Glenwood Bridge in Garden City. Wednesday morning, the gauge showed about 7,200 cfs passing beneath the bridge — roughly 10 1/2 feet of water.
Residents should expect to see Greenbelt flooding and possible river bank erosion. Minor flooding may also show up at Eagle Island and along other low spots near the river.
Managers may have to release more water into the river “in the coming weeks, depending on weather conditions,” warned a Wednesday morning news release.
Not since 1997 have dam managers had to raise the river above flood stage for so long. On Jan. 3, 1997, a rain-on-snow storm and warm temperatures increased the Boise Basin runoff, sending 148,000 acre-feet of water into the reservoirs in a matter of days. Officials took the Boise River up to flood levels soon after, and kept it high into July.
“Early March storms are expected to bring several feet of snow into the western and northern parts of the state, which is a reminder that winter isn’t over yet,” said Daniel Tappa, hydrologist with the U.S. Natural Resources Conservation Service in Boise.
The Boise River Basin is much like the entire state, which received from 150 percent of normal snow in February up north to 500 percent in the Big Lost River drainage.
The Payette and Weiser River basins received plentiful precipitation, too, 254 percent and 220 percent of average, respectively. Water year-to-date precipitation ranges from 120 percent to 160 percent of average.
The highest snowpack in Idaho can be found in the Big Lost, Fish Creek and Little Wood basins, which sit at nearly 200 percent of median. The lowest snowpacks are 90 percent to 110 percent of median in the Panhandle Region basins. Heavy snow years can produce heavy streamflow, so water users need to closely watch conditions in their specific basin, Tappa said
“There will be an extended high-water season when the snow starts melting; the timing and magnitude of peaks flows are unknown at this time, and the peaks could be big on some rivers,” Tappa and others who prepared the March Water Supply Report wrote.
The three Boise River reservoirs provide the U.S. Army Corps of Engineers and the Bureau of Reclamation critical storage for irrigation water and reducing the threat of flooding to billions of dollars worth of property within the Boise River’s historic floodplain. But dam officials can’t guarantee that they always can avert a major flood.
Warm temperatures and heavy rains anytime this spring could force dam managers to face a costly and difficult choice as in 2012. That year temperatures reached the 90s in late April, and managers briefly raised the river to 8,000 cfs.
In the spring of 1943, before Lucky Peak was built, warm temperatures in April sent 25,000 cfs of water over Arrowrock Dam and flooded parts of the Valley that today are filled with homes and businesses.
People interested in buying flood insurance need to research and decide early; the insurance doesn’t go into effect until 30 days after purchase.
Ada County is providing free sandbags to residents in need throughout the county. There is a limit of 10 bags per household, with the exception of Garden City, where residents are allowed 25 bags each. The city of Boise was not distributing sandbags as of Tuesday. Residents must fill their own sandbags and provide their own shovel.
Sandbags are available at the following locations:
Star: Star Fire Protection District, at 10831 W. State St.
Eagle: Eagle Fire Station No. 1 parking lot, at 966 Iron Eagle Drive.
Meridian: Meridian Public Training Center, at 1223 E. Watertower Lane.
Garden City: Garden City Library, at 6015 Glenwood St.
Flooding updates will be posted on Facebook and Twitter by Ada County and Ada County Emergency Management and at adacounty.id.gov.
Ada County Emergency Management can be reached at 577-4750.