By Vicente Calderón, WaterToday México Bureau.

At the U.S.-Mexico border, a river carries a little water and a lot of controversy.

The Tijuana River runs about 120 miles (193 kilometers) through Tijuana, a city of more than 2 million people, then crosses the border and flows about 5 miles (8 kilometers) through Southern California to the Pacific Ocean.

American Rivers listed it among the nation’s most endangered waterways in 2024, when it ranked 9th, and in 2025, when it jumped to No. 2, citing “out-of-control sewage and chemical pollution.”

This year the contamination keeps closing Southern California beaches and making people sick, even those who stay out of the water. South Bay residents are still demanding a solution.

“This community has waited decades for Mexico City and the administration to stop the sewage, and it’s still flowing,” said in their social media accounts Paloma Aguirre, a San Diego County Supervisor who made the issue central to her campaign.

U.S. politicians often accuse Mexico of not doing enough. John Beckham, director of the North American Development Bank, or NADBank, which finances environmental projects for the two countries, said the outcry is justified. But he said both countries are working on a complex problem and have invested millions of dollars.

“We have seen serious progress in the work on the sanitation infrastructure,” Beckham said. “While it isn’t complete, from a point where we had explosive growth and a collapse of the system, it has really recovered,” he said at a binational water forum at the Tijuana Cultural Center in September.

The crisis is usually told from San Diego’s side of the border. But Mexican citizens, researchers and academics are increasingly documenting the pollution at its source, measuring how it affects people on their side and pressing their own government.

The Baja California state government says it has invested nearly $70 million during the current administration to repair about 15 treatment plants, with money from the federal government and NADBank loans. Officials acknowledge the work takes time.

Tijuana activists have joined counterparts in San Diego even though water supply, not sanitation, is the most pressing issue for coastal Baja California. Tijuana, Playas de Rosarito and Ensenada have had water service cut during a hot summer on several occasions.

They depend on the dwindling Colorado River, which begins hundreds of miles away in the Rocky Mountains.

Three recent gatherings showed the growing Mexican involvement. 

Academics, activists and officials from three northwestern Mexican states met at the Autonomous University of Baja California in Tijuana to formalize the Northwest Water Network (Red Hídrica del Noroeste). 

According to a university news release, it aims “to strengthen knowledge generation and joint action in the face of water scarcity, pressure on supply sources and the effects of climate change.”

Days earlier, the University of California, San Diego (UCSD), hosted a watershed forum in Tijuana focused on sanitation and cross-border pollution. 

The river also came up at the first public session of San Diego Climate Week.

Proyecto Socioambiental Río Tijuana, a research group, surveyed 390 people in San Diego, Playas de Rosarito and Tijuana in 2025. Of those, 47% reported a respiratory condition, 16% eye allergies, 15% gastrointestinal problems and 11% ear infections, said Sara González one of the researchers.

Illness was common even among people who never touched the ocean, suggesting airborne exposure, said González. She stressed that the study is exploratory and does not prove cause, and that other factors, such as damp housing, may play a role. It was the first survey of people on the Mexican side focused on the problem, she said.

“It’s quite worrying that we find it isn’t only about going into the sea,” she told Water Today Canada.

UCSD researchers were among the first to show that the river’s pollution becomes airborne. A 2022 study found sewage bacteria and chemicals in sea spray, and a May 2025 study found cocaine metabolites and other chemicals aerosolizing from the water. An August 2025 study in the journal Science found hydrogen sulfide levels inland about 4,500 times typical urban levels.

San Diego Coastkeeper, a partner in the survey, continues to monitor the river on the U.S. side. Its executive director, Philip Musegaas, said bacteria levels in its samples “were extremely high.” He said industrial chemicals must also be treated at the source, before they reach the wastewater system.

González said the team also found industrial pollutants in wastewater, including heavy metals and PFAS, known as “forever chemicals.” That suggests weak oversight, she said: “a lack of institutional capacity to supervise that companies aren’t dumping into the drain pollutants they should be treating.”

The source of the discharges isn’t known, and because many U.S.-owned factories operate in Tijuana, she is wary of blaming only Mexico when transnational companies also benefit from lax environmental rules.

A solution still seems far off, but grassroots cooperation appears to be growing on the Mexican side, despite the large gap between the two countries’ economies.

Philanthropy is also stepping in. The San Diego Foundation recently announced $983,439 in grants for 10 projects involving 21 nonprofits in both countries. Five target the sewage crisis directly. The same initiative partly funded the health study.

“Water and pollution don’t stop at the border. Neither can our response,” said Pamela Gray Payton of the foundation.