Federal dam managers will raise flows on Boise River to 9,500 cubic feet per second by Tuesday, and the odds that flows will go higher than 10,000 cfs have risen to above 10 percent, they say.
Releases of 9,500 cfs will near the peak flow of 1983, the highest the river has been since Lucky Peak Dam was built in 1955. That will mean additional flooding in Eagle Island, Garden City, Star, Caldwell and Boise, with water over parts of Warm Springs Golf Course, Municipal Park in East Boise and more of the Greenbelt.
The National Weather Service is forecasting cooler temperatures for the next seven days, which will help lessen snowmelt, but a precipitation forecast of 1-3 inches of rain has hydrologists worried, said Chris Runyan, an engineer with the Bureau of Reclamation.
“The continued wet pattern just won’t let up,” Runyan told the monthly water supply meeting at the Idaho Department of Water Resources office in Boise.
Your questions answered with our Flooding 101: Why the river floods... and will again and again
Snowpack in the Boise watershed is 174 percent of normal, with 1.6 million acre-feet of water in the mountains, mostly above 6,300 feet.
“We still have most of the snow (at) 7,500 and up,” said Dan Tappa, a hydrologist with the Department of Agriculture’s Natural Resources Conservation Service. “There’s still a lot of water to come down.”
Flows coming into the three Boise River reservoirs were at 24,000 cfs early this week, said National Weather Service hydrologist Troy Lundquist. On Thursday, those flows were at 18,000 cfs – still a large number.
“We’re very concerned with how much snow is in the basin,” said Steve Hall, Walla Walla District Army Corps water management program manager.
The long-term forecast after May 24 calls for typically warm weather. Multiple days of 90-degree temperatures could melt the snow rapidly and create even higher flows.
In the past week, the reservoir system has lost more than 100,000 acre-feet of storage; it has just 182,000 acre-feet of storage space remaining with which to manage flows.
Many residents have asked why federal dam managers didn’t begin increasing flows to make space in the reservoirs earlier than Feb. 10. Runyan said that even though snow in the valley came and stayed in December, the snowpack was only slightly above normal in the mountains on Feb. 1.
February’s precipitation was then 300 percent of normal. Dam managers “started ramping up” to 7,000 cfs, which is flood stage, on Feb. 12, Runyan said.
A flow of about 9,500 cfs as measured at the Glenwood Bridge could affect some residential property near the river but should stay below most structures. The river will spill farther above its banks between 45th and 47th streets into the Garden City warehouse district.
Continuing erosion of riverbanks will be a significant problem, the Bureau of Reclamation and Corps of Engineers said, and quantities of floating debris collecting at bridges could cause additional flooding.
A large tree that got hung up in the river in Garden City last week acted like a dam, sending water onto 46th, 47th and 48th streets. The tree has since drifted downstream and hasn’t moved since Sunday, said Garden City Mayor John Evans. The riverbanks are too saturated to get heavy equipment close enough to fish out the tree, he said.
“We have over 5 miles of Greenbelt,” Evans said. “We aren’t in a position to attempt to protect private property. If there’s something within our resources that we can reasonably do along the riverbank, then we’ll see what we can do.”
Matt Whitlock, who restores classic cars in a building on 46th Street, said water didn’t get in last weekend thanks in part to jersey barriers put up by the building’s owner.
In addition to mandatory evacuations and road closures in parts of Blaine County, the county saw its first flood-related fatality on Wednesday.
Around 11:10 a.m., Blaine County dispatch responded to a report of a 54-year-old Ketchum resident yelling for help from a flooded basement. According to the Blaine County Sheriff’s Office, a rescue team was sent to 85 Eagle Creek Road, north of Ketchum.
The victim, Mike J. Wirth, was found unconscious in one room of the basement that was flooded with about 6 feet of water. Wirth was the owner of a landscaping company that was assisting the homeowners with flood-related issues. He was transported to St. Luke’s Wood River Hospital, where he died. The exact cause of death had not been determined as of Thursday afternoon.
Additional evacuations were ordered in the county on Thursday.
The Blaine County sheriff expanded a mandatory evacuation order for homes along Warm Springs Road. The new order included homes in the Della View subdivision, from the south side of Della Vista Drive and west of Red Elephant Drive, up to War Eagle Drive.
Once evacuated, residents will not be allowed to re-enter without signing a waiver. Roads were closed at Cedar Bend Drive, just east of the Draper Reserve entrancement. For a full list of road closures and evacuation instruction, residents may visit the Blaine County Sheriff’s Facebook page.
Flooding continues to pose problems in Blaine and Elmore counties as well.
Agencies in Ada County and beyond have been on alert in recent weeks. Residents of flood-prone areas are encouraged to keep informed of changing river, stream and weather conditions on the National Weather Service website. Residents also can follow flood-response instructions issued by emergency managers. Ada County residents can sign up for CodeRED emergency alerts.
For real-time Boise River flows, check the Bureau of Reclamation website.
Lucky Peak pool elevation will increase rapidly over the next week, so recreationists should take special care near the edge of the water, the agencies said.
Reporters Katy Moeller and Ruth Brown contributed to this report.