In south Napa, the lands east of Soscol Avenue have been home to two kinds of construction in recent months. One is the hard-to-miss work of humans, the other the hard-to-see and long-rare creation of beavers.
Thursday morning, a chain of vehicles inched onto the freshly laid parking lot of the Napa Crossing South center. Shoppers heading into the just-opened Marshalls store kept their eyes peeled for scarce parking spaces, never glancing north toward the neighborhood’s other construction site — a beaver dam local officials say has taken shape in nearby Tulocay Creek for more than two years.
A few hundred yards off the road, the creek’s waters slowed to a stop amid a grass-shrouded mound of branches and mud, forming an unexpectedly placid pool amid the strip malls and car lots. Two hundred feet upstream sat a mound of earth and twigs, and the willow trees from which the branches had grown — the telltale sign of a pair of beavers who have made this obscure stretch of water a home, for themselves and other wildlife.
“Further up the creek it’s dry and overgrown with trees,” Rusty Cohn, a Napa resident and frequent beaver watcher, said during a morning stroll along the bank. “Here you might see a large bass, or five or six turtles sunning themselves on a tree. It’s like an oasis here.”
Beavers have formed at least 20 dams on the Napa River and its tributaries, according to Shaun Horne, watershed and flood control resource specialist for the Napa County Flood Control and Water Conservation District.
“Especially in the last five years, we know they’ve (come to) Tulocay, Salvador, Sheehy and Fagan creeks,” he said. “We’re also aware of quite a few on the Napa River upstream, from Trancas Street all the way up to St. Helena.”
Many people have long seen the wood-gnawing, water-dwelling mammals as a nuisance whose handiwork kills riverbank trees and raises flooding risks on constricted streams and creeks. But the county’s flood control district, along with some local enthusiasts, instead see a sign of increasing health and diversity for urban waterways — and of the beavers’ continued revival in Northern California after fur hunting nearly exterminated them from the region by the turn of the 20th century.
In addition to creating wetland areas that harbor numerous plant and animal species, the beaver dams trap sediment, acting as a kind of filter to improve water quality in the Napa River system, according to Horne.
Beavers form the base of a dam by felling small trees and branches to create a base, then assembling the remainder of the structure from sticks, mud, rocks, leaves and other materials. The resulting backup of a stream raises the water level high enough to keep entrances to the animals’ den underwater, protecting themselves and their annual litters from predators.
The return of beavers to the Bay Area reached its peak of attention starting in 2007, when a mating pair dammed Alhambra Creek in downtown Martinez, formed a den and toppled trees the city had planted during a $9.7 million flood control campaign.
A proposal by engineering consultants to euthanize or relocate the beavers sparked an outcry from naturalists and residents, who formed the nonprofit group Worth a Dam to spare the water dwellers and call attention to their benefits. Eventually, the city spared the beaver family, which has produced at least 19 offspring since, according to Heidi Perryman, founder and president of Worth a Dam.
“We’ve seen improvements in our creek,” she said Friday. “We see otter, steelhead, wood ducks, turtles, even mink, all because of habitat the beavers make”
No such human-vs.-beaver conflict appears imminent around Napa, Horne said while viewing the Tulocay Creek dam, where the water level of the resulting pond is about 6 feet below an adjoining hotel’s parking lot.
Despite the animals’ reputation for choking waterways, Horne said the county flood district generally restricts its intervention to annually surveying streams and removing thicker fallen trees, or surrounding others with wire to shield them from gnawing. Cattails and other vegetation are considered a higher risk for increasing silting, and the district trims back cattails and prunes some willows every two years.
“Generally we leave them alone,” he said. “Usually, beaver dams will break up when you have high enough flows, and then the beavers come back and rework the sites again.”
Meanwhile, Rusty Cohn could take a long look at the beavers’ handiwork and, for a few minutes, put himself in an emotional place far from the busy shops nearby or the semitrailers rumbling past.
“Here we are, a mile from downtown Napa, and I can walk 15 minutes from my house and see this,” said Cohn. “That’s kind of special.”