NORTH COUNTY -- San Mateo Creek, one of Southern California's last free-flowing streams and most suitable places for bringing back the endangered steelhead trout, has been named the nation's second-most threatened river.

San Mateo runs 16 miles from its headwaters in the Cleveland National Forest of Southwest Riverside County to San Onofre State Beach along the North San Diego County coast. The creek's largely intact habitat, the state park it winds through and the world-famous Trestles surfing area that depends on its cobble deposits are threatened by proposed construction of the Highway 241 toll road, a conservation group warned today in a report listing the nation's 10 most endangered rivers.

"The health of the creek is the reason all of these things exist," said Mark Rauscher, assistant environmental director for the Surfrider Foundation in San Clemente.

Rebecca Wodder, president of the 65,000-member American Rivers group, said by telephone Monday that the coastal stream is in danger of being destroyed for a project of questionable benefit.

"We're trading something that is irreplaceable for something that is basically just a big parking lot," Wodder said.

However, Jennifer Seaton, spokeswoman for the Transportation Corridor Agencies, the road builder, said the project would complete an important missing link in Orange County's toll highway system while avoiding damage to the creek by filtering rain runoff.

"The TCA has a strong record of building roads and minimizing their impact on the environment," Seaton said. "We value the same resources that the report talks about. We believe that the project can be built to relieve traffic and at the same time protect those resources. Our position is that you can have both."

American Rivers, which disagrees the two can coexist, says only the Santa Fe River in New Mexico is in greater jeopardy. In its 22nd annual report, it also cites rivers in Washington, Iowa, Wisconsin, Texas, Arkansas, Pennsylvania, North Carolina and Alaska.

"These are 10 rivers that are at a crossroads, rivers that within the next 12 months are going to see decisions that will determine their future for better or for worse," Wodder said.

The national conservation group is advocating that agencies come to their aid by rejecting a proposed power line, writing rules to clean up waste water and taking out an existing dam.

In San Diego County, the organization wants regional, state and federal agencies to deny permits to the toll-road builder. Over the next year or two, the builder will be seeking approvals from the San Diego Regional Water Quality Control Board, California Coastal Commission, Federal Highway Administration, U.S. Army Corps of Engineers, California Department of Fish and Game and others.

Wodder said the group's track record suggests there is a chance its effort to block completion of the last 16 miles of the toll-road system will be successful.

She said, for example, that within 24 hours of listing the Susquehanna River -- the longest waterway on the East Coast -- as an endangered river in 2005, a proposal to downgrade its existing protections was withdrawn.

George Sutherland, steelhead project coordinator for Trout Unlimited, a fishing organization, praised the decision to list San Mateo.

"That's way cool," Sutherland said. "This will bring it to the attention of a lot of people."

Mark Capelli, steelhead recovery coordinator for the National Marine Fisheries Service in Santa Barbara, said the San Mateo is one of the most promising streams for bringing back the southern steelhead, an ocean-dwelling rainbow trout. Capelli said that's because of the creek's long run on public land -- from the protected San Mateo Canyon Wilderness to Camp Pendleton Marine Corps Base to the state park along the coast.

Also, Sutherland said the creek has no natural or manmade barriers that block the fish's ability to migrate upstream to spawn and an estuary that is in reasonably good condition.

Said Rauscher: "It's worth trying to protect because it's the last one -- the last natural watershed in Southern California."

Rauscher said the toll road would dilute efforts to protect the creek by compromising its water quality. Spilled oil and fuel, broken chunks of oil-based asphalt and stripped pieces of tire rubber would wash into San Mateo from the highway when it rains, he said.

Seaton, of the Transportation Corridor Agencies, said the road would not degrade the water. She said an extensive system would be constructed to collect rain and filter pollutants before runoff reaches the creek.

Not only that, she said, the agency plans to build such a system along two miles of Interstate 5 where unfiltered water now washes into the creek.

According to a study prepared by Encinitas engineer David Skelly in 2000 for the agency, the highway would not change surfing conditions at Trestles Beach.

Contact staff writer Dave Downey at (760) 740-5442 or ddowney@nctimes.com.