SAN DIEGO - Border Field State Park occupies some 400 acres in the very southwestern corner of the continental United States. The entrance to the park isn't exactly inviting. There's an empty dirt lot for parking and a gate that's usually closed.
But a project is underway to make the park more welcoming, using trash collected from the adjacent Tijuana River Valley and estuary.
On a recent morning, Steven Wright was leveling off a row of plastic soda bottles embedded in a layer of wet concrete. When finished, the structure will be a bench for park visitors.
Wright is the co-founder of the organization 4Walls International, which takes trash found in the border region and repurposes it as construction materials to build homes and infrastructure.
"We actually sourced these out of the canyon, right upstream," Wright said of the plastic bottle building blocks, each stuffed full of plastic bags and discarded Styrofoam.
4Walls pays Tijuana residents 5 pesos a stuffed bottle, about $0.40. Kids and teens living in the canyons that feed into the Tijuana River watershed have been especially enthusiastic recyclers, Wright said.
"A 14-year-old showed up the other day with like 140 bottles and we had to pay him out," he said.
The Tijuana River starts south of the border, in the mountains east of Tijuana, and ends in the Pacific Ocean just north of the U.S.-Mexico border. The estuary formed where the river meets the sea is internationally recognized for its biodiversity and ecological importance.
It is Southern California's only remaining intact coastal lagoon.
About 70 percent of the Tijuana River watershed is in Mexico.
"And a lot of the socioeconomic issues on the south side of the border have a direct correlation with some of the environmental issues here on the north side of the border," Wright said.
The work to improve the Border Field State Park entrance is funded by a grant from the San Diego Foundation. It's part of a larger effort to mend the border environment in the U.S. and in Mexico, and to involve the people who stand to benefit from cleaner water and a healthier Tijuana River Valley.
September and October are prime clean-up months in border canyons and river valleys, because soon the rains will come, bringing trash, polluted water and sediment from illegal dumps and construction sites in Tijuana.
From then until spring, people usually can't even get in to the park because trails and roads get flooded. And the waters are often too toxic to do cleanup.
"As a result, a large section of the community is not getting the experience of this river valley," said Chris Peregrin, director of the Tijuana River National Estuarine Research Reserve, which includes Border Field State Park.
"And I believe that if people are not experiencing it than they're also not going to advocate for it," Peregrin said.
Just south of the border, dozens of people have been cleaning up an abandoned park stretching along a narrow canyon in the Playas de Tijuana neighborhood. The canyon feeds into the Tijuana Estuary on the U.S. side, and is nearly hidden from the surrounding, busy streets.
The workers here are employed on a temporary basis through a program run by SEMARNAT, Mexico's equivalent of the Environmental Protection Agency.
Here, too, 4Walls International is turning trash into benches, with support from the U.S. EPA.
"This place is important because, first, Tijuana doesn't have many green areas or places to recreate," said Ana Eguiarte, a Tijuana resident and community outreach coordinator for the Tijuana River National Estuarine Research Reserve.
"And, second, the idea is to preserve this area, to recover the original vegetation," she said. The park, called Los Sauces, is Tijuana's first conservation area.
In the northern portion of the park, native willow trees - the park's namesake - form a dense riparian forest, nearly blocking out sounds from the surrounding neighborhood.
Homeless people and deportees used to camp here, Eguiarte said, but local authorities recently kicked them out so the workers could start restoring the natural environment.
The thick, rust-colored steel bars of the U.S.-Mexico border fence run straight across the canyon. Beyond, the canyon empties out to the Tijuana Estuary in the U.S.
José Palomera Chavez is one of the workers employed by SEMARNAT to clean out the canyon. He said he was grateful for the job, but he seemed somewhat skeptical of his neighbors' will to keep areas like this clean.
"The truth is that we don't do our part," he said. Palomera Chavez lives in a neighborhood of Los Laureles canyon, which also feeds into the Tijuana Estuary.
"I've noticed that several times they've cleaned the canal in my neighborhood, and you should go see it now, it's full of trash, polluted water and tires," he said. "If it's not cleaned up again, and it rains hard, all of that trash is going to end up here."
In some ways, this is exactly the point of these binational restoration efforts - to make people like Palomera Chavez aware that how you treat your neighborhood makes a difference not just there, but everywhere downstream.
U.S. border residents can do their part for the Tijuana River and estuary at a community cleanup day at Border Field State Park on Oct. 12.