By Emily Stifler

In the early part of the 20th century, the Shields River Lower Canal Company built a four-foot tall, 10 inch wide concrete dam to provide water for an agricultural channel in the Shields River valley north of Livingston. Today, the Chadbourne Diversion still provides vital irrigation to working farms and ranches in the valley. But it’s in need of repair.

Over the years, the dam has also protected the migration and growth of the native Yellowstone cutthroat trout upstream. By limiting the incursion of rainbow, brown and brook trout into that watershed, it protected the native cutthroats’ habitat and genetic purity. Although brown trout and brook trout are threats to Yellowstone cutthroat trout in the Shields River watershed, they remain relatively secure because of the protection from rainbow trout.

As a result, the Shields remains a basin level stronghold for Yellowstone cutthroat trout. “That means we’ve got an intact watershed where the fish can move freely, not just isolated populations,” said Carol Endicott, a fish biologist with the FWP.

“We have 375 miles of interconnected stream that still support Yellowstone cutthroat trout. We don’t have that anywhere else in Montana,” she added.

Rainbows, which are abundant in the river below the diversion, pose the biggest threat to cutthroat because the two interbreed. If the diversion were to fail, it would open the “floodgates” for rainbows to move upstream, something that would be catastrophic for the cutthroat population in the Shields, Endicott said.

Endicott, working with another FWP biologist, Scott Opitz, and the agricultural shareholders in the Lower Shields River Canal Company, have received grant funding to repair and retrofit the Chadbourne Diversion. The structure would cost $1 million to replace, Endicott said.

“This project melds the interest of irrigated agriculture with fish conservation,” Endicott said. “The canal fed by the diversion delivers water to 13 farms and ranches – it’s essentially the lifeblood of their ranching and farming operations.”

As it stands now, during high water irrigators have to manually add headboards to back up the flow behind the structure to send water down the ditch. “It’s dangerous work,” Endicott said. “We’re looking at find a way to come up with a mechanical alternative that would not be so unsafe.”

And this spring, the stakes on the repair project were raised: With the high runoff, massive cottonwood trunks floating downstream knocked an eight-foot wide chunk out of the concrete, said Mike Dailey, a nearby landowner who holds Shields water rights. Dailey is Secretary and Treasurer of the Lower Shields River Canal Company, which currently maintains the diversion and is working on a temporary fix to the damaged part of the dam.

While repairing the structure following the irrigation season in 2012, Endicott’s team will first make the barrier impassable to fish swimming upstream. Then, they’d like to build a fish ladder with a “stock pen.”

During the trout spring migration season, FWP workers will sort the fish. If they’re cutthroat, they can go above the dam. If they’re rainbows, they’ll be returned to the river below. Other native species will also be passed over the dam, which will restore their historic migratory access.

While this isn’t the first time the FWP has built barriers to protect native fish, this project is unusual in that the barrier will have selective passage, Endicott said.