Boise & Garden City

‘Nobody controls a flood.’ So what does Boise River Flood Control District No. 10 do?

Flood Control District #10 tour of the Boise River

Take a look at some flooding worry spots, from inundated gravel pits to bridges that are uncomfortably close to being under water,
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Take a look at some flooding worry spots, from inundated gravel pits to bridges that are uncomfortably close to being under water,

Boise River Flood District No. 10 might spend the next three years pulling out all of the trees that have fallen into the river this spring or are leaning so badly they soon will, said Bill Clayton, chairman of the flood control district’s board of commissioners.

That’s just for the area inside the district’s boundaries, which stretch from the Plantation Golf Course near State Street in Boise to just east of the Interstate 84 bridge over the river in Caldwell, Clayton said.

As federal water managers prepare to raise the Boise River to its highest flows since 1983 Tuesday, there are likely more trees, debris and other challenges ahead for the district’s crews to handle this spring. Clayton guessed crews will find hundreds of downed or compromised trees by the time the flooding stops.


Flood 10 — the shorthand people who work for or with the district use — is a taxing district that the state of Idaho organized in the early 1970s. Its mission is to protect life and property along the Boise River by maintaining the river channel.

Most of Flood 10’s efforts fall into three categories: removing trees and other debris from the river; stabilizing the riverbanks; and removing gravel that could alter the channel and lead to more flooding. Landowners are responsible for maintaining the riverbanks that touch their property, but the district works with them to develop plans and get necessary permits when they need to repair them.

“We are somewhat misnamed,” Clayton said. “It really should be a river maintenance district rather than a flood control district. Nobody controls a flood. We might think we do, but we don’t.”

Debris removal is the main focus these days. It’s a tricky job, because crews can work only in safe conditions, which usually means between November and February when the water is low. The trouble is, with so much water rushing down the river this spring, there’s more debris getting caught up.

If trees or other objects get hung up on the supports of a bridge, the debris can restrict river flow and worsen the flooding. In a worst-case scenario, water can rise enough to undermine the bridge and cause it to fail, said Mike Dimmick, Flood 10’s manager. Electrical and other utility lines often run along the sides of bridges, so keeping them above the water level is a priority.

Usually, the transportation agencies in charge of bridges remove debris that poses an immediate threat, Dimmick said. The district focuses more on long-term maintenance of the river.


Though flood control is the district’s named mission, working in or around the Boise River is always a balancing act.

It is the Treasure Valley’s gem, a place for people to sight-see, fish, float, ride bikes and generally enjoy. Clayton recognized this in the early 1990s. Not wanting the river to turn into a Los Angeles-style concrete ditch as the Treasure Valley grew up, Clayton started Boise River 2000 with Karl Gebhardt, a veteran hydrologist and environmental engineer whose company, Resource Systems, is based in Boise.

Boise River 2000 was an initiative whose goal was to create a master plan for the Boise River to inform cities along its channels on which areas are appropriate for building and which ones aren’t. The effort didn’t yield a master plan.

“That’s still my dream,” Clayton said.

But some good did come out of it. Clayton said the work that he, Gebhardt and others put into the project started a conversation between a variety of groups, many of whom are natural competitors. Environmental groups, anglers, irrigators and developers started talking to each other about their concerns instead of nursing them in isolation, Clayton said.


Over the past three to four years, the district itself has become more sensitive to the values of groups whose interests sometimes conflict with its own, said Joe Kozfkay, Idaho Department of Fish and Game’s southwest regional fisheries manager.

Kozfkay credited Dimmick and Steve Sweet, a hydrology and engineering consultant who does a lot of work for Flood 10, for the change. Both men are avid anglers, a fact Kozfkay thinks encourages them to think about how the district can carry out its responsibilities while protecting fish populations in the Boise River.

“Before both of their arrivals, we had a pretty rough relationship with the flood control district,” he said. “We thought that they were way too aggressive and really didn’t take fisheries or habitat concerns into account. And since Mike and Steve have both been there, we think that’s much improved.”

Tree removal is one area where Flood 10’s cultural change is especially pronounced, KozfKay said. Generally speaking, trees are good for fish, especially trout. A thick canopy of trees over the water keeps it cool, which trout like. Trees also provide habitat for bugs, which fish eat. Roots that stick into the water create pools where fish can hide out. Roots also stabilize river banks, reducing the amount of sediment that gets into the water, where it can clog the spaces between rocks that newly spawned fish use for cover.

People, too, like shady, tree-lined banks to stroll, cycle, fish or contemplate the river.

These days, Flood 10 is a lot more careful about removing trees, Kozfkay said. Sometimes, if a tree is leaning and looks like it will fall soon, the district will remove some branches to make it lighter and keep it in place instead of just cutting it down.

“Before, any time a tree started to lean, they used to define it as a hazard tree and then they’d just remove it,” Kozfkay said.

Saving trees isn’t the only way in which Flood 10 has become more conscientious about its operations, Kozfkay said. Before the days of Sweet and Dimmick, he said, the district did some things that are hard to believe in today’s context.

“In the old days, they would think nothing of taking an excavator and just running it in the river channel for hundreds of meters if not miles to get to another site,” he said. “That’s not done anymore.”


As challenging as this year’s high water has been for Flood 10 and its fellow flood experts and professionals, it’s provided an unusual opportunity to improve knowledge of the Boise River’s channels and flooding models.

The district hired Gebhardt to conduct what’s known as 2-D modeling along the river. The 2-D concept has been around for decades, but modern computing power has made it more practical because it can more quickly and conveniently process enormous piles of data.

Unlike the traditional practice of one-dimensional modeling, which measures how much water is flowing down a channel at a given time and uses that to calculate where water will go as river rises, 2-D modeling measures volume, velocity and direction of water flow through a grid. Those calculations are layered on topographical information for each box inside the grid.

The idea is that all this additional information will more accurately predict where water will go at varying levels in the waterway. Also, it predicts what will happen as water levels recede after a flood — which areas will continue to be under water and which ones will dry out.

This year’s flooding has given Gebhardt a chance to calibrate his model. So far, he said, it hasn’t needed much tweaking. The first calculations the program yielded — at a group of gravel pits near West Boise — predicted that water at its current flow rate would be within two inches of elevation of its actual level, he said.

The district also hired the University of Idaho to identify high-water marks along the river. Clayton hopes the district can use data from Gebhardt’s 2-D modeling and the university’s measurements to move into a more proactive approach.

“And maybe the communities can look at that and decide where they should put development and where maybe they should cut development off,” he said. The goal, Clayton said, is to use science “to help other people make better decisions.”

Boise River Flood Control District No 10 at a glance

Boise River Flood Control District No. 10 is one of 18 flood control districts in Idaho.

Its boundaries run along both sides of the Boise River and Dry Creek between Garden City and Caldwell. About 25,000 acres of land and 40 linear miles of waterway are inside the district.

The district collects taxes on private properties inside those boundaries. It has a yearly budget of about $300,000 and employs three people: a full-time manager, Mike Dimmick, a part-time bookkeeper and a part-time consultant. Private contractors do much of the work the district is responsible for.