Caltrans is completing plans to replace a small bridge on a major thoroughfare — Highway 121 linking south Napa and Sonoma counties — while keeping traffic running without a detour.

The $13.9 million Huichica Creek bridge project could begin construction in 2020 and last for two years. The stated goals are to make the highway safer for motorists and to make the creek easier to navigate for fish.

Caltrans intends to demolish an old bridge, build a new one and still allow an average of 32,000 vehicles daily to keep crossing the creek under Highway 121, which is also known as Highway 12 and the Carneros Highway. The idea is to do the project in phases, with the new bridge overbuilt initially and the extra width removed during the last phase. Traffic will shift as a section of the new bridge is built and a section of the old bridge is removed.

Nighttime work is to be minimized for environmental reasons. Daytime motorists and bridge construction workers will have to co-exist.

“There is going to be a little slowdown to improve the highway,” Caltrans spokesman Vince Jacala said.

Caltrans released a 164-page report on the proposed Huichica Creek project bridge at www.dot.ca.gov/d4/envdocs.htm, which is still in the design phase.

“Everything isn’t set in stone yet,” Jacala said.

Two-lane Highway 121 runs through the rolling, vineyard-covered hills of the Carneros region. A new Huichica Creek bridge will be the final piece of a bigger project that improved virtually all of a 1.7-mile stretch from Duhig Road in Napa County to the Sonoma County line.

This section once saw higher rates of fatal accidents than comparable highways, a Caltrans report said. In 2011, Caltrans finished widening the road, adding shoulders and smoothing curves. Accident fatalities dropped from eight from January 2001 through December 2003 to none from July 2012 through June 2015, statistics show.

But Caltrans didn’t replace the Huichica Creek bridge along this stretch of Highway 121 to fit in with the rest of the wider roadway. The reason was proposed fish passage improvements associated with the proposed bridge didn’t satisfy various state and federal environmental agencies, officials said.

Huichica Creek flows for about 16 miles from the southern Mayacamas mountains to Napa Slough, which empties into the Napa River. Steelhead trout are in the stream. The state Department of Fish and Wildlife considers the portion of the creek at the bridge to be a fish barrier to upstream spawning grounds.

Downstream of the bridge is a six-foot to eight-foot drop that fish have trouble passing. The creek crosses under the bridge in three 78-inch-wide metal culverts built in 1968, when environmental laws were laxer.

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“It’s like a broken leg of the stream,” Jacala said.

A new-and-improved Huichica Creek bridge would have no culverts, but would be a free span bridge clear of the creek. Caltrans proposes to remove paved portions of the creek channel near the bridge, create a gentler slope in the channel and build eight step-pools that fish could travel between with a maximum half-foot jump. Fish passage improvements are to extend 300 feet downstream of the bridge and 130 feet upstream.

“Water will flow better and it will be a better watershed,” Jacala said.

The new bridge will even allow more light to penetrate to the bottom of the creek. That should allow for natural physical and biotic conditions, the Caltrans report said.

Meanwhile, motorists will see an altered landscape. Workers will have to remove oaks, sycamores, Oregon ash, white alder and other trees to complete the project, perhaps as many as 100, though Caltrans said it will make efforts to avoid or prune trees instead.

But motorists won’t see the loss of one of Napa County’s famous, historic stone bridges. The Huichica Creek bridge has no charming stone sides, but metal guard rails.

The new, concrete bridge is to be 44 feet wide, nine feet wider than the current version.

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Napa County Reporter

Barry Eberling covers Napa County government, transportation, the environment and general assignments. He was worked for the Napa Valley Register since fall 2014 and previously worked 27 years for the Daily Republic of Fairfield. He is a graduate of UC Sa