By The Numbers: What Happens As The River Rises To Each New Flooding Level

Apr 12, 2017

As dam officials bump up the water flow on the Boise River yet again this week, it’s a good time to take a look at the numbers that matter during this flooding event.

This week, the Bureau of Reclamation and the U.S. Army Corps of Engineers plan to raise the water level at the Glenwood Bridge to 8,500 cubic feet per second. As of Wednesday, crews were pushing 9,240 cfs of water out of Lucky Peak Dam. Gina Baltrusch with the Walla Walla District of the Corps says about 1,000 cfs is being diverted into irrigation canals and the rest is flowing down the Boise.

“Right now we’re managing flows,” Baltrusch says.

Here’s the problem. The three reservoir Boise River system, which includes Anderson Ranch, Arrowrock, and Lucky Peak dams, can only hold 949,700 acre feet of water. But the system is rapidly filling up. Just 295,224 acre feet of room is left. And, as of April 1, there was still 2 million acre feet of water in the snowpack that feeds the system. That’s a lot.

“It’s a very unusual year,” says Baltrusch.

Hot weather and/or too much rain could put more strain on the reservoir system. The goal is to empty enough water out now, to be able to take in more as the snow melts.

“It’s a delicate balance and we are constantly getting new data,” says Baltrusch.

While 8,500 cfs at Glenwood is the new target, Baltrusch says they may have to release more water, depending on weather conditions.

At 8,500 cfs, more of the Greenbelt will be underwater. And Baltrusch says some basements may flood near the river as the water table rises. Shorelines will become unstable, she says, and unsafe.

But what happens if the river does go higher?

We've seen what happens when the Boise River starts to creep above flood stage, which is 7,000 cfs at the Glenwood Bridge. The National Weather Service has a prediction chart, of the likely effects if the water keeps going up. Here's a step-by-step look:

4000 cfs

Or 8.5 feet, water will begin seeping onto low spots on the Greenbelt path next to the river especially near bridge underpasses.

6500 cfs

Or 9.9 feet, large sections of the Greenbelt path adjacent to the river will be submerged. Minor flooding may affect portions of Eagle Island.

7000 cfs

Or 10.2 feet, large sections of the Greenbelt path adjacent to the river will be submerged. Erosion of river banks will also become a significant problem. Debris in the river including small trees and logs may begin piling up at bridge crossings. Minor flooding will be observed on sections of Eagle Island and in other low spots near the river. Minor flooding will spread downstream to low areas near the river in Star and Caldwell.

8900 cfs

Or 11.2 feet, large sections of the Greenbelt will be submerged and severe bank erosion will occur. Flooding will occur in low areas close to the river in Boise, Garden City, Eagle and Caldwell, but the water should remain below the level of most residential areas.

9500 cfs

Or 11.5 feet, moderate flooding of low areas near the river in Boise, Garden City, Eagle, Star and Caldwell will occur. Debris in the river will collect on bridges crossing the river. The water may affect some residential property near the river, but should stay below the level of most residential structures.

10500 cfs

Or 12.0 feet, flooding near the river will occur in low areas of Boise, Garden City, Eagle, Star and Caldwell. Portions of Eagle Island will be submerged with flooding of residential property in low-lying areas near the river likely. The water should remain below the level of most residential structures, but access in and out of some neighborhoods may be limited by high water.

12000 cfs

Or 12.5 feet, flooding will occur in low areas and parkland adjacent to the river in Boise, Garden City, Eagle, Star and Caldwell. Flooding of low-lying residential streets in areas adjacent to the river is likely, especially on portions of Eagle Island. Debris damming on bridges crossing the river will occur and may cause water to spill over the road surfaces leading up to the bridge.

16000 cfs

Or 13.3 feet, major flooding will occur near the river in Boise, Garden City, Eagle, Star and Caldwell. Some residential areas in low-lying areas near the river will be flooded with water entering some houses. A flood of this magnitude has not occurred on the Boise River since 1943. Many city parks near the river will be underwater. Debris piling up on bridge crossings will cause additional flood damage.

How much higher will the water go? It's hard to say, and a lot depends on the weather. A few hot days could melt the snowpack too fast, while colder, dryer weather would lead to a more moderate melt. Here's one number we've seen, from the Idaho Statesman on Monday:

"Some Idaho water experts say privately that the river could have to be raised to as much as 12,000 cfs for a month to prevent a week of flows as high as 20,000 cfs," -The Idaho Statesman.

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