Two-thirds of rivers in the eastern United States are becoming increasingly alkaline, making their waters more dangerous for crop irrigation and fish life, scientists said Monday.

Even though alkaline is the opposite of acid, the reason for the change is the legacy of acid rain, which eats away at rocks and pavement that are high in alkaline minerals, said the researchers in the journal Environmental Science and Technology.

"It's like rivers on Rolaids," said lead author Sujay Kaushal, a geologist at the University of Maryland.

Researchers examined 97 rivers from the northeastern state of New Hampshire down to Florida over the past 25 to 60 years.

These rivers are important because they provide drinking water to big cities such as Washington, Philadelphia, Baltimore, Atlanta, and other major metropolises.

They found "significant increasing trends in alkalinity at 62 of the 97 sites," said the study.

None of the rivers studied had grown markedly more acidic over time.

Researchers said higher alkaline content in the water can complicate wastewater and drinking water treatment and lead to faster corrosion of metal pipes.

Water with higher alkaline levels can be harder, saltier and contains more minerals than soft water.

It can also lead to ammonia toxicity that can harm irrigated crops, as well as fish and other river life.

Alkalinity increased the fastest in places where carbonate or limestone rocks lay beneath bodies of water, at high elevations, and where acid rainfall or drainage was high, the researchers said.

A process known as chemical weathering -- whereby acid eats away at limestone, other carbonate rocks and even sidewalks -- is blamed for dissolving alkaline particles that wash off into waterways.

"In headwater streams, that can be a good thing. But we're also seeing antacid compounds increasing downriver. And those sites are not acidic, and algae and fish can be sensitive to alkalinity changes," said Kaushal.

Even though acid rain is on the decline in the United States, due in large part to tighter environmental restrictions put in place in the 1990s, its legacy is continuing.

"This is another example of the widespread impact of human impacts on natural systems (which) is, I think, increasingly worrisome," said study co-author and ecologist Gene Likens of the University of Connecticut and the Cary Institute of Ecosystem Studies.

"Policymakers and the public think acid rain has gone away, but it has not."

Researchers said it is difficult to predict how long this trend of river alkalinization will persist.