This is the time of the year when you don’t have to be a farmer or a boater to appreciate the dams on the Boise River.
The Boise River would be sending nearly 16,000 cubic feet per second of water through town today if Arrowrock, Lucky Peak and Anderson Ranch Dam weren’t here. Of course a lot of us wouldn’t be here either because it was irrigation that turned Boise from a mining supply town to a regional center and state Capitol.
As it is, the Boise River is running at nearly 7,000 feet at Glenwood Bridge, the lower end of flood stage, when people on Eagle Island begin to get their feet wet. Bu it will take a lot more rain and hot weather to push the river up to 16,600 cfs, the level when the dams no longer hold back the flows.
Right now the flows are within the Corps of Engineers and Bureau of Reclamation’s “rule curves” the probability-based guides dam managers use to determine when a reservoir will fill. This year is running at about the same rate as 1984, said Brian Sauer, a hydraulic engineer for the Bureau of Reclamation in Boise. Thee rule curves give the engineers the tool they need to control the river as far as they can.
With 266,000 acre-fee of space left in the three reservoirs, the Corps and the Bureau, which manage river flows jointly, will be able to keep the more than 1.6 million acre-feet of water that has flowed and will continue to flow down the Boise River, from inundating people’s homes. But a few factors can change conditions so that the agencies are forced to allow the flood waters to flow.
The one we have to worry about right now is more heavy rains along with hot weather. If we get both for more than a few days the river is going to come up. Hotter weather melts the snowpack faster than the reservoirs can catch it and dam managers will keep flooding, albeit at the minimum they have to keep control of the river.
In 2002, a
University of Idaho flood simulation was developed that laid out where flood waters might end up. They didn’t follow the flood plain map in place at the time.
Part of the reason is that when the map was drafted developers whose land was shown to be in the flood plain immediately got approval to build and to haul in dirt to lift the homes above the 100-year flood level, which is where engineer expect to flood 1 percent of the time.
These changes meant that when the water comes where it goes will be unpredictable. But a couple of cottonwood trees could be swept into the river and catch on the Glenwood Bridge, creating a natural dam, the Glenwood dam.
If we get through the next few weeks of peak runoff without major flooding the next period to watch is in early July when managers are close to filling the reservoirs. They try to get it just right but again, a hot spell can bring down the last of the snow melt and they can lose control of the river, forcing them to release flood flows.
This is never any easy decision. But one only has to look at the Mississippi, where the Corps of Engineers has opened flood gates to inundate small towns to save the big cities, to see they will do what's best for the most people when forced.