The flows on the Boise River have been rising this week and are expected to reach 6,000 cubic feet per second through the city, a level just under flood stage.
More than 2 million acre-feet of water is stored in the three upstream Boise reservoirs and in the snow covering the 2,680 square-mile watershed. That means Treasure Valley residents can expect high flows into June and beyond.
Lucky Peak Dam, and to a lesser degree Anderson Ranch and Arrowrock dams, provide the U.S. Army Corps of Engineers and the Bureau of Reclamation critical storage to reduce the threat of flooding to billions of dollars worth of property within the Boise River’s historic floodplain. But dam officials have been frank with residents that they can’t guarantee they always can avert a major flood.
“We have gotten very good at working with the Bureau, but we recognize the finite capacity of the system,” said Steve Hall, Army Corps water management program manager.
The damage to the Oroville Dam in California drew national attention to the vulnerability of some emergency spillways. But that is not the issue at Lucky Peak Dam.
Flooding on the Boise River is not tied to the spillway on top of Lucky Peak, as many residents have surmised since the Oroville Dam’s spillways nearly eroded. The Army Corps, which built Lucky Peak in 1955 and owns and operates it, has the capacity to release 30,000 cfs — five times current flow through Boise — out of its outlet tunnels. That’s enough to protect the dam, but also enough to cause a major flood in the Boise River Valley.
“That’s not something we want to do, but we have that capacity,” Hall said.
Jan. 3, 1997, came the closest to a major flood since Lucky Peak’s construction. A rain-on-snow storm and warm temperatures increased the inflows, putting 148,000 acre-feet of water into the reservoirs in a matter of days. Officials took the Boise River up to flood levels, and kept it high into July.
Had another 10,000 acre-feet came into the reservoirs behind the three dams, they would have been filled and officials would have had to release the rest, causing a major flood.
The releases and high flows this week will make room in the three reservoirs upriver from Boise for the huge spring runoff. Hall and his partners at the Bureau, working with state and local officials, will keep raising flows if more snow piles up in the mountains or if rains or warm weather fills the reservoirs more quickly. The reservoirs are now 61 percent full.
The extent of development in the historic Boise River flood plain has reduced the capacity of the channel to move water downriver. One part of the problem is that some developers built structures to keep the water out of their subdivisions. That causes flooding in other areas downstream.
And then there is chance. A log could become lodged at Glenwood Avenue Bridge. Other obstructions could divert flood water into an unanticipated direction. And at Eagle Island, with its expensive homes, the Boise River flows split around the island differently every year.
“The south channel has significantly less capacity than the north channel,” said Mary Mellema, Bureau of Reclamation supervisory hydrologist in Boise.
A Boise River flow of 7,000 cfs is considered flood stage. Raising the river above that begins to become costly in terms of damage to riverside properties.
Hall and Mellema have faced that decision before. In May 2012, rains and hot weather caused runoff to accelerate and forced officials to raise Boise River flows to 8,000 cfs.
That level damaged irrigation canal headgates and sent water over roads and the Boise River Greenbelt. The river came inches from homes on Eagle Island, though none were flooded, Hall said. But that doesn’t mean they take the decision of raising the flow to 8,000 cfs lightly.
“I don’t think we’re ever going to say 8,000 cfs is a safe thing,” he said.
MORE DATA, BUT MORE UNPREDICTABILITY
Dam managers make these decisions using runoff forecasts and snow measurements with “rule curves,” well developed predictions that define how much reservoir space that must be empty to catch high spring runoff. The agencies monitor snowpack by satellite, then factor in history and instinct to decide how much water they need to run through the river.
But predictions have become harder, as runoff begins earlier and climatic conditions have become more variable due to global warming. At the same time, the dam managers have more data today — and a great record of keeping flooding to a minimum.
“One thing we found out: The rule curves on the Boise are very adaptable, recognizing that things are changing and very variable,” said Mellema, the supervisory hydrologist.
With temperatures having cooled off to below the late-February average, those conditions should help delay the runoff for at least a couple of weeks, said Troy Lindquist, a hydrologist with the National Weather Service in Boise. The chance that temperatures and precipitation will be above normal, normal or below normal are about equal, he said.
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, which was then mostly rural.
Today, a week of 80-degree temperatures in mid-April, or a cool spring that delays runoff into June when reservoirs are supposed to be full of irrigation water, could send a torrent of water through Lucky Peak such as Boise hasn’t seen in 70 years. Temperatures in April average in the 60s, but in 2012 they reached the 90s, Lindquist said.
BALANCING FLOOD CONTROL, IRRIGATION
Flood control isn’t the only issue. Managers have to balance protection against having the reservoirs full for irrigation.
That’s not much of an issue in February, but come June when the reservoirs are nearly full, it takes a deft touch on the part of Hall and his partners to fill the reservoirs without going over.
Fred Shoemaker is a lawyer who lives in the River Run neighborhood along the river in southeast Boise. He watches the river closely and knows the flood risk. He’s confident he would be the last house in his neighborhood to flood, but he’s bought flood insurance two out of the last 12 years.
He’s thinking about buying it again this year. Flood insurance doesn’t go into effect for 30 days after property owners buy it.
“The real systemic problem on the Boise River is that the dams are multi-purpose, so it’s a real challenge for the people who have their hand on the lever,” said Shoemaker. “It would be complicated enough if we only wanted to provide flood control.”
Remembering the flood of 1862
One flood is not in the record books because it came before officials kept records, but is a cautionary tale nonetheless.
Boise historian Merle Wells told the story of homesteader Isaac Coston, who reported that on July 4, 1862, all the land in the river bottoms “extending from bluff to bluff and from the present site of Boise westward to the canyon near the present site of Caldwell” was under water.
Engineers estimate that the river that summer ran at 100,000 cfs, 16 times the flow today.