As soon as it rains, coho salmon likely will swim up a small creek to a spawning ground in the west Sonoma County hills that has been impassable to the species for decades.
When they come to the culvert where Purrington Creek flows under Graton Road about 3 miles southwest of Graton, the endangered coho will get a boost, literally, from a concrete fish ladder set in the narrow streambed.
The $366,000 project, completed by Sonoma County crews in November, will make it "easy for even the weakest fish" to reach a nearly mile-long stretch of the creek upstream from the culvert, said Luke Walton, a hydraulic engineer who designed the project.
It's the latest improvement in a two-decade, multimillion-dollar effort to improve Sonoma County coho habitat and restore the population of a prized wild fish that once flourished along the North Coast but teetered near extinction in 2000.
Up to 500,000 coho spawned in Northern California rivers in the 1940s, dwindling to about 5,000 by the mid-1990s.
Purrington Creek, a rare year-round waterway, flows into Green Valley Creek, the last of the streams in the 1,485-square-mile Russian River watershed known to support a significant coho population.
"Purrington's got a lot of good things going for it," said Derek Acomb, a state Department of Fish and Wildlife planner for the local watershed.
A 2003 study of 125 stream crossings at Sonoma County roads identified the concrete box culvert under Graton Road as a "high-priority" obstacle to the coho, which swim up to 150 miles from the ocean to lay eggs in gravel streambeds.
Purrington was on a "short list" of fish passage problems, but it took years to secure the funding for a fix, Acomb said.
Enabling coho to swim through the culvert will open up 4,700 feet of potential spawning habitat in the midst of an 887-acre watershed.
Damage to stream habitat and barriers like the culvert are major factors in the coho's precipitous decline, Acomb said.
Aesthetically speaking, the project designers would have preferred to strategically fix boulders in the streambed below the culvert, said Mike Jensen, the project manager for Prunuske Chatham Inc., a Sebastopol firm that specializes in ecological restoration.
But only a bare concrete structure about 40 feet long would fit there without trees being cut down, he said.
The structure's four chambers, with a notched wall between each chamber, allow the coho to make a series of 1-foot jumps to reach the culvert.
It also raises and slows the flow of water through the culvert, said Walton, the project engineer. Adult coho need about a foot of water for easy passage, he said.
Steelhead trout, which also spawn in Purrington Creek and have been spotted upstream from the culvert, are a more athletic fish that can swim faster and jump higher than coho, Acomb said.
Sonoma County Transportation and Public Works crews built the structure with rock riprap along the streambank high enough to prevent damage from a 100-year-flood.
Fish were removed from the creek and a bypass pipe was installed to carry water around the construction site, said Dave Dammuller, a Department of Transportation and Public Works engineer.
Willows will be planted among the rocks to shade the creek and maintain the cool water temperature that coho favor.
Coho salmon begin spawning in November and December and, unlike steelhead, die after laying and fertilizing their eggs. Juvenile coho remain in fresh water for a year, then head out to the ocean for a year or two before making their lone spawning trip.
So far this season, 25 hatchery-tagged coho have been confirmed in the lower Russian River, below Wohler Bridge, and an estimated 150 in all are in the river.
Most of the coho are trapped in the river's main stem, because access to all the tributaries -#8212; except for Dry Creek near Healdsburg -#8212; is cut off by low water or no water in the streams, said Mariska Obedzinski, a fishery biologist with California Sea Grant.
A trickle of clear water flows through the new fish ladder on Purrington Creek, and officials are awaiting rainfall that will send the coho surging up the waterway.
"All we need is a good storm," Acomb said.
(You can reach Staff Writer Guy Kovner at 521-5457 or guy.kovner@pressdemocrat.com.)
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