Flooding on Boise River, why now?
With six times as much water as Lucky Peak can hold sitting as snow in the mountains above, the people who manage and live on the Boise River are facing unpleasant decisions.
Federal dam managers are raising river flows to 8,500 cubic feet per second next week, as measured at the Glenwood Bridge. With two months of runoff season left, officials say they could be forced to raise the river higher than it’s gone since Lucky Peak was built in 1955. The highest flow was in June 1983: 9,500 cfs.
“We need to move as much water as possible down the river,” said Steve Hall, U.S. Army Corps of Engineers water management program manager. “We are trying to raise the river without doing much damage.”
Already many portions of the Greenbelt are underwater in Boise, Garden City and Eagle, and the river has widened into lakes that cover hundreds of acres of cottonwood groves through Canyon County to the edge of Caldwell.
Think the flooding won’t affect you? At somewhere between 9,500 and 11,000 cfs, water would be running over Eagle Road at the Boise River’s south channel bridge. At 10,500 cfs, the river would spread over ParkCenter Boulevard.
In Eagle, the river crept into the parking lot at Bardenay restaurant. Nevertheless, diners ate on the patio Wednesday evening. Children played in nearby Merrill Park, just off the river’s edge and the closed Greenbelt. In the Island Wood and Rivers End subdivisions on Eagle Island, residents were mowing lawns. Contractors were building new homes.
But if flows rise to 9,500 cfs or above, those subdivisions could see water surrounding homes and closing roads, said Doug Hardman, director of Ada County Emergency Management. Somewhere between 9,500 and 11,000 cfs, water would begin running over Eagle Road at the Boise River’s south channel bridge and force the closure of the state’s busiest nonfreeway road.
High releases could last into summer.
‘THERE ARE MANY UNKNOWNS’
Hardman bases his predictions on simulation software provided to Ada County by the U.S. Army Corps of Engineers and the U.S. Weather Service. Using the same predictive software, Hardman uses the U.S. Weather Service forecasts that at 9,500 cfs, water would cover portions of Warm Springs Golf Course and Municipal Park in East Boise.
The river, which already has spilled its banks between 45th and 47th streets in Garden City, would rise into the warehouse district and some residential property could be flooded. At 10,500 cfs, the river would spread over ParkCenter Boulevard.
And that’s without some entirely plausible emergency, such as uprooted trees. Such simple, natural events could force flooding even at today’s levels, warns Hardman.
“All it would take is for a very large tree to jam against a bridge. That could cause some homes significant flooding,” Hardman said. “This is a natural event and there are many unknowns.”
In fact, one official confided, federal managers could be forced to raise the river to 12,000 cfs for a month to prevent flows of 20,000 cfs for a week.
‘PIT CAPTURE,’ AND A TWIST
Perhaps the biggest question is whether the Corps of Engineers team can stop the “capture” of the Sunroc gravel pit just south of Eagle Island. A team is working this weekend to construct a temporary levee to shore up one side of the pit so it doesn’t cave and allow the river in.
If the river were to flow into the Sunroc pit, that would change the course of the river and send water in directions experts now can only guess. Corps officials say modeling shows it could cause flooding for homes, businesses and subdivisions downriver and to the west.
Pit capture also could flood the nearby Boise Watershed wastewater treatment plant off of Joplin Road.
Hall said officials are optimistic that the levee can hold, but there is one other piece of the puzzle. Sinkholes were found as a result of flowing water in the gravel pit. These “voids” mean that there might still be a route for the river to get past the levee to fill and capture the pit.
“We don’t know how many voids there are under the surface,” Hall said.
SIMPLE FLOOD MATH
The reason this has been so hard to manage is that the March flows into the three Boise River reservoirs — Arrowrock, Anderson Ranch and Lucky Peak — were more than 300 percent higher than average, setting a record. It’s simple math: Between 10,000 cfs and 16,000 cfs flowed into the reservoirs, while dam managers were releasing 9,000 cfs and less.
Despite the high releases into the Boise River, there’s less room in the reservoirs now than there was in February. On Thursday, the flows coming into the reservoir system were 10,000 cfs.
The worst-case scenario would be to have multiple days of 80-degree weather and rains that turn the 2 million-acre feet of snow in the Boise watershed into water rather quickly, when reservoirs have just 320,000 acre-feet of space, said Mary Mellema, Bureau of Reclamation supervisory hydrologist in Boise.
“I don’t know if any one event that we would pinpoint would fill all that storage right away,” she said. “Our concern would be with additional rainfall and warming temperatures melting the snow, that would cause a big spike in the inflows so the runoff would come really rapidly.
“Right now we’re trying to release as much water as we can that doesn’t cause a large amount of damage.”
LEARNING FROM 1983
In 1983, such an event came in June, shooting flows into the reservoirs at 24,200 cfs. That forced dam managers to raise the river to 9,500 cfs at Glenwood, more than it had ever taken since Lucky Peak was built.
Dam managers make these decisions using runoff forecasts and snow measurements with “rule curves,” well-developed predictions that define how much reservoir space 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.
“We’re trying to take a measured approach to release as much water as we can,” Mellema said. “There are times we’re going to be above the rule curves. That’s just how things happen. ... But we don’t try to flood people just to get to the rule curve unless it’s necessary and we’re running out of space.”
More snow is possible in Idaho’s mountains this weekend. And that means more challenges for the people responsible for getting through this spring without causing massive economic effects and costly damage to public and private property, Ada County’s Hardman said.
“This is going to be a long-term fight for sure,” Hardman said.
Editor’s note: This story has been updated to correct an inaccurate description of Sunroc’s location and flows at Eagle Island.
Some observant Boiseans noticed that a few manhole covers on low-lying streets and areas near the Boise River Greenbelt have plugs in their holes.
The answer is pretty simple. The city wants to limit the amount of floodwater entering its sewer lines so that Boise’s two sewer treatment plants aren’t overwhelmed, said Rob Bousfield of the Public Works Department.
If enough floodwater dripped through enough manhole covers, Bousfield said, it could cause the treatment plants to fail. That’s why the city put cork stoppers in the holes of some covers. Some of the those covers are submerged, Bousfield said.
“It doesn’t have to be entirely drip-proof or anything like that,” he said. “We just want to keep the majority of the water out.”