The recent discovery of hundreds of young coho salmon in a tributary of the Russian River near Jenner is being hailed by biologists as a breakthrough in the decade-long effort to restore the critical habitat and nurse the endangered fish back to health.

Approximately 450 coho were counted in the upper reaches of Willow Creek this summer, an astounding number given that virtually none of the fish have been seen in the waterway for the better part of two decades.

Run-off from logging and farming, coupled with the end of dredging efforts that were aimed at preventing road flooding, had turned the nearly-nine mile waterway flowing from Coleman Valley to the Jenner estuary into a meandering mess.

But restoration work that involves numerous government agencies and nonprofit organizations, and to date has cost more than $1 million, appears to be paying off, to the degree that Willow Creek is quickly becoming one of the healthier habitats for coho among all 150 creeks and streams that comprise the Russian River watershed.

"It's really exciting," Derek Acomb, a California Fish and Wildlife scientist, said last week while he observed workers push massive redwood and fir logs at selected junctures of the creek, the latest in a series of projects aimed at restoring the habitat.

Acomb, who used a waterproof camera to capture a remarkably clear video of several young coho swimming in the creek in August, recalled saying out loud to himself, "Wow!" when he first spotted the fish. The video has become a minor hit with his colleagues on YouTube.

Michele Luna, executive director of Stewards of the Coast and Redwoods, called the sight of so many coho in Willow Creek a "huge triumph" for all of the agencies involved in repairing the damaged fishery.

Acomb's discovery was especially noteworthy because it means the young coho were spawned in the creek by fish that traveled approximately seven miles upstream from the Pacific Ocean to the pool in Coleman Valley where the fish were spotted. The journey would not have been possible prior to the restoration work, including the remodel of a 43-foot bridge that was a barrier to the fish making it upstream.

Biologists consider Willow Creek arguably the most important tributary habitat in the Russian River for coho, Joe Pecharich of the National Oceanic and Atmospheric Administration said. The federal agency has contributed about $400,000 for the restoration work.

He said young coho, as well as steelhead trout, are using the lower half of Willow Creek to feed and grow before they migrate to the ocean, as biologists had hoped.

"So not only have these projects opened passage and provided rearing habitat for Willow Creek, they have also provided for any Russian River salmonids that choose it," Pecharich said.

Government biologists have released more than 33,000 young coho into Willow Creek since 2011. The first of those spawning fish, which have tiny wire tags implanted in their snouts, or in some cases, transponders for more advanced tracking, are not expected to return to the creek until later this fall.

Acomb said a coho released in Willow Creek was caught by a commercial fisherman in Oregon, revealing how far the fish can travel before returning to their spawning grounds.

In the meantime, work continues on making Willow Creek a more hospitable place for the fish. Crews are putting wood into strategically placed locations of the creek to create a more complex stream channel, one that will have larger and deeper pools, as well as cover to protect salmon from river otters, herons and other predators.

Much of the wood was cleared out of the stream in the 1970s and 80s when Louisiana Pacific operated in the watershed, according to John Green, of the Gold Ridge Resource Conversation District. Green said that created a creekbed of uniform width and flatness, or what he termed a "bowling alley" during a tour of the creek last week.

The Gold Ridge district, California State Parks and Fish and Wildlife are now working together to put 100 fir and redwood logs into the stream. Much of the wood is being donated by the Mendocino Redwood Company, which owns property in the area, as well as by two San Francisco residents whose ranch includes a section of the waterway.

Specific areas along a 2.5-mile section of creek have been mapped out for placement of the logs. Last week, Ken Smith of Willits lifted a 43-foot log using a skitter equipped with a grappling hook and shoved it down an embankment as project managers looked on.

For Smith, a former contract logger, the task of putting wood back into the creek carries some irony. He said he was never convinced that taking the wood out of creekbeds in the first place was a wise idea.

"We all lived in the creek when we were kids. Granted, there was a lot more wood in the creek then, but taking it out sterilized the creek," said Smith, who owns Pacific Inland, Inc.

The debris project, which also is being overseen by Blencowe Watershed Management out of Fort Bragg and is funded with a $160,000 grant from Fish and Wildlife, is supposed to wrap up by Oct. 15, so as to not interfere with the mating season of red-legged frogs, Green said.

"Instead of truckloads of wood going out of here, we're having truckloads brought in, which is probably the first time that's ever happened," he said.

(You can reach Staff Writer Derek Moore at 521-5336 or derek.moore@pressdemocrat.com. On Twitter @deadlinederek.)