A brown trout fry.

BOISE — A Boise River project aims to improve trout habitat and counter some of the impact of development in southwest Idaho.

Volunteers with Boise Valley Fly Fishers have been working with the Boise River Flood Control District No. 10 to enhance habitat in a side channel, adding small gravel ideal for spawning and creating cover with woody material.

The district works on the river each year between irrigation seasons, clearing debris and making other improvements. With Lucky Peak, Arrowrock and Anderson Ranch dams capturing gravel from mountain streams above them, less gravel is deposited in side channels, Mike Dimmick, the district’s project manager, said in a release.

The New Dry Creek side channel “is working well as a ‘test plot’ where we can experiment with gravel placement and woody material,” he said.

State Department of Fish and Game surveys found about 85 brown and rainbow trout fry in the side channel compared to about 20 in other locations surveyed.

“That site is one of the most productive areas in the Boise River,” said Troy Pearse, Boise Valley Fly Fishers conservation director. “It’s good evidence that our work is showing positive results.”

Urbanization means the river must be managed more tightly, he said in an interview. For example, the flood control district removes trees to reduce flood risk and cities remove wood to increase safety for people who float the river — both of which leave fish with less cover from predators.

The district partners with Idaho Fish and Game to improve woody cover without increasing flood risk, Pearse said. “They can do that in side channels because it’s less likely to move to areas where it could become a flood risk.”

Fish and Game fry surveys show Trout Unlimited gravel augmentation has worked in the upper river, in east Boise, increasing the productivity of that reach, he said.

Downstream, the reach between Glenwood Bridge in Garden City and the head of Eagle Island in Eagle is less productive — underperforming the rest of the river from Lucky Peak Dam east of Boise downstream to Star to the west.

The flood control district, which uses heavy equipment, can augment gravel faster than hand crews. The district, in cooperation with Fish and Game and the Boise Valley Fly Fishers, “is working to proactively change its policies on woody debris to improve trout habitat,” Pearse said.

An increase in adult brown and rainbow trout is expected in around five years in the project area, he said.

About 10 volunteers with the Fly Fishers group worked with Flood 10’s contractor to spread 15 cubic yards of gravel in the side channel. Sunroc, of Eagle, donated the gravel, placing it in the upper end to allow currents to spread it out downstream when the river rises.

This is the project’s third year. Last year, they placed about 12 yards of gravel in the side channel and anchored woody material along the bank for winter cover.