The Idaho Water Resource Board on Sept. 21 approved a flood-management grant of $85,340 to help Twin Falls Canal Co. build a sizable wetland connected to East Perrine Coulee.
The project aims to reduce flood risk to Twin Falls neighborhoods downstream as well as the amount of sediments and pollutants coming off farmland and winding up in the Snake River, said Twin Falls Canal Co. General Manager Brian Olmstead.
East Perrine Coulee, a natural stream that predates irrigation canals, “is mainly a drainage system for several thousand acres of farmland,” he said. “We deliver water to all of these farms, they irrigate with it, and it returns to this low coulee and then on through the City of Twin Falls and into the Snake River Canyon.”
The coulee is a flood risk because it has limited capacity and drains a large area. Olmstead said. And the amount of water entering it probably increased, he said, as fewer and larger farm fields — many irrigated with center-pivot sprinklers and draining in one direction — replaced collections of smaller fields, each with its own cross ditches and drainage areas.
Excess water from the coulee in the past generally flowed to pasture land below and did not cause much damage. But residential development over the past 20 to 30 years, he said, increased the percentage of ground at risk of flood damage, as subdivision land tends to absorb less water than cropland.
“This is really a large expansion of a small wetland we started,” Olmstead said.
The initial project, a 3-acre pond installed in the mid-2000s, “helped a lot, but (East Perrine Coulee) still gets overwhelmed during big events,” he said. Major flooding occurred late in the heavy 2016-17 winter.
At completion, the project is expected to have around four times the amount of water storage and drainage capacity, and be equipped to handle around 90 percent of flood events, Olmstead said.
Construction is expected to start by mid-October on the Kimberly Road site, about two miles east of Twin Falls and half a mile north of the Chobani yogurt-manufacturing complex, he said. Chobani, which built its own runoff catchment system, is upstream and will not be impacted.
The project’s costs, Olmstead said, are $388,000 to buy about 27 acres — farmland the coulee roughly bisects — from the late Don Norris and his sister Betty Schmidt, and $204,000 to build additional ponds, maintenance and dirt stockpile areas.
The 15 bullrush-rimmed ponds are essentially wetland cells through which water flows before it enters the large pond on the downstream end, according to a site map. Check structures will divert water from the coulee into each cell.
The wetland is expected to catch sediments and nutrients from agricultural lands, a valuable aspect, Olmstead said.
The U.S. Environmental Protection Agency’s National Pollutant Discharge Elimination System stormwater program addresses non-agricultural sources. Thus ag’s compliance is voluntary.
“We are building water quality projects all the time to stay proactive and ahead of regulations on discharges to the river,” Olmstead said.
The property is valuable because of its proximity to Twin Falls and the Snake River Canyon, he said.
“This was the last place to build a wetland that anybody could afford to build a wetland on,” Olmstead said.
Twin Falls Canal Co. irrigates 202,000 acres, all south of Snake River Canyon, and serves many farmers.
Also Sept. 21, the Idaho Water Resource Board approved flood-management grants of $115,460 to Nez Perce Soil & Water Conservation District and Nez Perce County toward fixing an erosion choke point along Bear Creek next to a road that provides the only access to the town of Peck; $26,105 to the city of Pocatello to help with stream bank stabilization and flow control on part of Pocatello Creek; and $6,025 to Riverside Village in Garden City to help in repairing a damaged diversion structure, the board said in a news release.