LOMA MAR — A 77-year-old concrete dam is now gone from San Mateo County’s Pescadero Creek, opening a route for rare coho salmon to once again return to their ancestral habitat.

The dam created a popular swimming hole at Memorial Park in the redwood forests of the Santa Cruz Mountains, where kids floated on air mattresses in the chilly waters and searched for crayfish.

But it prevented the fish from making their swim from the Pacific Ocean to freshwater pools to spawn. The coho was listed as endangered in 2005 and is now on the brink of extinction.

The dam was demolished in October by a team led by the San Mateo County Resource Conservation District and San Mateo County Parks Department — and December rains have restored the creeks natural contours, opening up 62 miles of creek habitat to the fish.

“We’re trying to give them a fighting chance, where they come to spawn,” said Kellyx Nelson, executive director of the San Mateo County Resource Conservation District, based in Half Moon Bay.

“We’re doing everything in our power to make sure they have the right habitat to feed, grow fat and lay their eggs before going back out to sea,” she said.

The dam was built in 1938 to provide a swimming hole near the confluence of Peterson and McCormick creeks, part of the Works Progress Administration of President Franklin D. Roosevelt’s New Deal to combat unemployment in the Great Depression.

But it created an insurmountable barrier for fish in the coastal creek. The dam wall was too high for fish to jump when traveling upstream. And its narrow chute forced a fierce flow of water, too strong to overcome. And it prevented the downstream flow of gravel, essential for fish to lay eggs. An additional problem — also now fixed, as part of this restoration project — was created by a 1930s-era road crossing, designed to provide vehicle access to the park’s Sequoia Flat Family Camp.

“These are great projects which benefit fish — the result of many years of planning, layers of initiatives,” said Marlene Finley, director of the San Mateo Parks Department.

Once Pescadero Creek — whose name is Spanish for fish merchant — teemed with coho salmon, as well as steelhead trout. While steelhead are still found in Pescadero Creek, coho were believed to have vanished from its waters.

Coho, also known as silver salmon, are born in cold freshwater streams like Pescadero Creek, where they live for a year before swimming to the ocean. Then they return to lay eggs and fertilize them.

Last winter, to great excitement, biologists spotted three coho salmon that had recently spawned, including a 26-inch female. Those adult fish are believed to have originated in Santa Cruz County’s Kingfisher Flat Hatchery on Big Creek, a project of the National Oceanic and Atmospheric Administration and state fisheries.

But no fry were detected.

This is the fifth barrier to be removed by the San Mateo County Resource Conservation District since 2007. The project received about $280,000 in funding from the California Department of Fish and Wildlife, with additional funding and effort from the conservation district and the San Mateo County Parks and Recreation Foundation.

Of previous projects, the most formidable was the 2013 removal of a century-old 40-foot-wide concrete barrier from a section of the creek in El Palo Alto Park on the Palo Alto-Menlo Park border, restoring the streambed to a more natural course.

As winter rains deposit necessary gravel and cause the newly restored creek to swell, biologists will watch closely for the fish to return.

“This is the time of year that we might start to see fish migrate up creeks,” said Nelson. “I’m hoping.”