Pollution has changed the chemistry of rivers in the eastern United States, according to researchers.

In a survey of rivers from Florida to New Hampshire, scientists found that two thirds of all rivers in the region are alkaline.

The study looked at the alkalinity trends among 97 rivers over 25 to 60 years. The rivers had indeed either become alkaline or increasingly alkaline over time.

None became more acidic over time.

The cause is believed to be the legacy left by acid rain, which, counter-intuitively raises the level of alkalinity.

Researchers say that despite acid rain being curbed by effective government policy, its legacy has lasting effects on the environment.

"This is another example of the widespread impact humans are having on natural systems. Policymakers and the public think that the acid rain problem has gone away, but it has not" said study author and ecologist Gene Likens, in a statement.

"Acid rain has led to increased outputs of alkalinity from watersheds and contributed to long-term, increasing trends in our rivers. And this is twenty years after federal regulations were enacted to reduce the airborne pollutants that cause acid rain."

Increased alkalinity in rivers affects drinking water and sewage treatment. It also increases the growth of algae.

The more alkaline the water, the more there is also a risk of ammonia toxicity, which can affect wildlife and food production near rivers.

The most alkaline rivers were found near Washington DC, Baltimore, Philadelphia and Atlanta - rivers which supply those cities' drinking water.

The findings were published in the journal Environmental Science and Technology.