The study, published in the journal Nature on Wednesday, found that river mineral mining that involves intensive excavation and sediment processing in river corridors is altering river forms and releasing excess sediment downstream. This is causing siltation to levels that may result in disease and mortality in fish, as well as poor water quality and damage to human infrastructure.

The Dartmouth team looked at information collected from satellites over 37 years to see how mining is changing rivers around the world. They found 396 places where mining is happening in 49 different countries, mostly in warm regions. The rivers in these areas have been changed significantly by the dirt and rocks from mining.

Of the 173 rivers affected by mining, 80% of them have more dirt in the water than they did before mining started. In 30 countries where mining affects large rivers (more than 50 meters wide), about 23% of the length of these rivers has been changed by the dirt from mining. This adds up to 35,000 km of rivers, which is about 6% of all the large tropical rivers in the world.

"For hundreds, if not perhaps, thousands of years, mining has been taking place in the tropics but never on the scale like we've seen over the past two decades," said first author Evan Dethier, an assistant professor at Occidental College. "The degradation of rivers from gold and river mining throughout the tropics is a global crisis."

Mining has caused suspended sediment concentrations to double at 80% of the 173 rivers represented in the study relative to pre-mining levels.

"These tropical rivers go from running clear either throughout the year or at least through part of it, to either being choked with sediment or muddy year-round," says Dethier. "We found that almost every single one of these mining areas had suspended sediment transmitted downstream, on average, at least 150 to 200 kilometers (93 to 124 miles) from the mining site itself but as much as 1,200 kilometers (746 miles) downstream."

"To give you an idea of how far the sediment can travel downstream, this is nearly comparable to the distance from Bangor, Maine, to Richmond, Virginia," says Dethier.

There are 30 countries that have both active river mining operations and large tropical rivers that are more than 50 meters wide. In those countries, on average, 23% of the length of their large rivers is affected by mining. In some countries, more than 40% of the total length of those large rivers is altered by mining, including in French Guiana (57%), Guyana (48%), and Côte d'Ivoire and Senegal (40%).

The study also included rivers such as the Congo in Africa, the Irrawaddy in Asia, the Kapuas in Oceania, and the Amazon and Magdalena in South America.

"Many of these tropical rivers systems are very biodiverse places, if not some of the most biodiverse places on Earth and are still currently understudied," says senior author David Lutz, a research assistant professor of environmental studies at Dartmouth. "The challenge here is that there are many species that could potentially become extinguished before we even knew that they existed."

The study's findings highlight the need for better regulation of river mining activities to protect these important ecosystems.