Environmental activist Erin Brockovich has accused U.S. authorities of lying about how much toxic wastewater spilled from a Colorado mine and fouled rivers in three Western states.

Brockovich, made famous by the Oscar-winning movie in which she was portrayed by Julia Roberts, sensationally claimed 'millions and millions of gallons' was spilt.

Her allegation came during a visit to the nation's largest American Indian reservation, where she saw the damage and met with Navajo Nation leaders and farmers affected by last month's spill.

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Environmental activist Erin Brockovich visits the San Juan River, which is located downstream from the spill

Brockovich, who was made famous by the Oscar-winning Julia Roberts movie bearing her name, chats with Navajo Nation Vice President Jonathan Nez beside the San Juan River

Brockovich told students at Shiprock, New Mexico (pictured), the EPA 'did not tell the truth' about the amount of toxic water spilled into the rivers

Kayakers in the polluted river near Durango right after the spill in August

In this August 11, 2015, aerial photo, waste water is seen streaming out of the Gold King Mine near Silverton, Colorado

Brockovich said she was shocked by the agency's actions leading up to the release of waste tainted with heavy metals, as well as its response afterward.

'They did not tell the truth about the amount. There were millions and millions of gallons,' she said while speaking to a crowd of high school students in Shiprock, New Mexico.

The spill was triggered by an EPA crew during excavation work. The agency did not immediately respond to email and telephone requests for comment yesterday.

Although it initially pegged the spill at one million gallons, it later said it was likely three times that amount, given the readings of stream gauges that recorded a spike in river flows.

The revision only added to the suspicion of local officials that were criticizing the agency for failing to notify them sooner that the contaminated plume was headed downstream.

Uncertainty lingers over the long-term dangers to public health and the environment from the spill, which contaminated rivers in Colorado, New Mexico and Utah.

The EPA said the threat has eased, allowing treatment plants to start drawing water from the rivers again and ending warnings against recreational activities. But Navajo leadership is skeptical.

A series of congressional hearings investigating the spill will begin today. Republican committee leaders in the House and Senate say that EPA officials have withheld documents that could explain what went wrong.

Navajo President Russell Begaye also questioned the number of gallons released.

He recounted for Brockovich what he saw during an unannounced visit to the Gold King Mine near Silverton, Colorado, days after the spill.

He described seeing a wide gully that was several feet deep and water continuing to pour out of the mine.

Some Navajo irrigation systems remain shuttered until the tribe receives results from its own water and sediment testing. As a result, Begaye has said thousands of acres of crops have gone dry.

Begaye and Brockovich met with farmers to discuss the effects of the spill on irrigation as well as the legacy of contamination left behind by decades of uranium mining.

Pictured is actress Julia Roberts from a scene in the 2000 Oscar-winning film Erin Brockovich

Roberts (left) won plaudits for her portrayal of Brockovich (right), who helped build a case against Pacific Gas and Electric Company for contaminated groundwater

During the stop in Shiprock, they told the students that it will be up to the next generation to hold government and private industry accountable.

'It's a terrible disaster, and unfortunately it's a situation we see playing itself out not only on the Navajo Nation, but across the United States of America,' Brockovich said, referring to pollution and lax enforcement. 'You are the future and you will be the answers,' she told the students.

Brockovich was portrayed in the 2000 movie, 'Erin Brockovich,' which earned actress Julia Roberts an acting Oscar.

The environmental advocate helped investigate a major case of groundwater contamination in California in the early 1990s that inspired the film.

As for the Gold King spill, Brockovich said the federal government needs to clean up the mess.

Navajo officials say the Federal Emergency Management Agency and the EPA have declined the tribe's requests for continued help, including the appointment of a federal recovery coordinator.