Mexican officials have known the Santiago is heavily polluted for many years. In 2008, an 8-year-old boy, Miguel Ángel López Rocha, fell into a tributary of the Santiago. He scrambled out, but by dinnertime he was convulsing and vomiting. He died days later, of arsenic poisoning caused by the river, according to the National Human Rights Commission.

His death turned national attention to the river’s pollution, and the state commissioned a study. That 2011 report, by the Mexican Institute for Water Technology, found that the river contained high levels of arsenic, lead, cadmium, cyanide, mercury and nickel.

Two years later, a commission set up under the first North American trade agreement studied pollution in the Santiago and the adjoining Lake Chapala at the request of local communities. It found monitoring and enforcement failures as well as little evidence of an “alleged implementation of an ecological restoration plan” for the region.

But that trade deal, known as Nafta, did not allow for any penalties.

In 2017, the state of Jalisco, together with the National Autonomous University of Mexico, studied the river again and found its condition to be “critical,” with levels of many pollutants that repeatedly violated the permitted limits.

“The Santiago River is, for me, one of the most shameful, most terrible stories that Jalisco and Mexico have,” said the state governor, Enrique Alfaro.

Just after taking office a year ago, Mr. Alfaro visited the bridge over the waterfall that has become the symbol of the river’s pollution, and promised to tackle the problem — a bold pledge, given that both his power and his resources are limited.