Muddied tropical rivers reveal magnitude of global gold mining boom: Study

  • A new study reveals that 59,000 kilometers, or nearly 37,000 miles, of tropical rivers have been damaged by mining, based on 7 million measurements taken from satellite images spanning four decades.
  • The new study reveals a runaway river mining boom starting in the mid-2000s, largely caused by a tremendous spike in gold prices since the 2007-2008 financial crisis. Since then, the price of gold — an element which accounts for 90% of river mining in the tropics — has only gone up.
  • An estimated 5-7% of the world’s large tropical rivers are considered severely affected by sediment from river-mining activities, with 173 large rivers impacted in the global tropics.
  • Up to a third of global freshwater fish species are found in tropical rivers. Mining, much of it illegal or informal, seriously impacts both human and natural communities in numerous ways. Sediment traps aquatic life in dense, cloudy, muddied water, with sunlight unable to reach plants and fish.

Untold numbers of people go daily to tropical rivers worldwide to eke out livelihoods and seek their fortunes, as they dig up streambanks and pump riverbeds, creating swirling clouds of mud in a search mostly of gold, but also diamonds and other precious metals.

These efforts range from small to large-scale. Some miners hack away at riverbanks to reach mineral-rich soil, felling trees and uprooting vegetation vital for healthy rivers and causing severe erosion and runoff. Floating dredgers, sometimes funded by criminal syndicates, use massive hoses to suck up mineral-rich silt and filter it for hidden treasure, before pumping the murky water back into streams.

While conservationists on the ground have long warned of the surging amount of ecological harm being done, a new study quantifies the morphing problem. A satellite image survey spanning four decades and recently published in Nature is the first to illustrate the bigger, long-term picture.

A satellite image of the Madre de Dios region shows two rivers coming together: one mined and the other unmined. The bright caramel color helped researchers identify mining activity on a global scale. Image courtesy of Miles Silman/Arizona State University GCDS/Planet Labs.

Across the tropics, river mining exploded in the last 10-15 years at an unprecedented scale, the study finds. Debris from the predominately small-scale operations turned 173 large rivers in 49 countries a bright caramel color, making their heavy sediment flows clearly visible from space.

Up to 7% of large tropical rivers, defined as those wider than 50 meters (164 feet), are now severely impacted by mining, a total of 35,000 kilometers (21,750 miles), the researchers found. Another 24,000 km (14,900 mi) of smaller rivers are also impacted. Added up, the total distance would extend a fifth of the way to the moon.

The study’s lead author, Evan Dethier, a geomorphologist at Occidental College, was spurred to map tropical mining impacts after seeing the severe environmental harm inflicted on the Madre de Dios region in the Peruvian Amazon. He and his co-authors wondered what other rivers were seeing similar damage, but flying under the radar.

“It’s awe-inspiring to see how pervasive [river mining] is everywhere,” Silman told Mongabay in a phone interview. Before doing the research, “I had no idea that we were mining every river in the tropics and increasing the sediment loads, with effects on both the freshwater biodiversity — which in the tropics is comparable to biodiversity on the land — and the humans living in those areas.”

A satellite image of Madre de Dios shows two rivers coming together.
A research expedition in September 2019 to the Madre de Dios region in the Peruvian Amazon observed firsthand the extent of river damage due to gold mining. Image courtesy of Jason Houston/CINCIA.

Human and aquatic life suffocated

Mining doesn’t only cause rivers to change color; muddied streams also increase in volume due to the huge amounts of sediment added to their flow. In 10% of the mining-impacted rivers measured by the study, the amount of sediment in the streams increased tenfold. In 80%, the sediment had at least doubled.

Murky waters dim the sunlight vital to supporting the base of the food chain in these aquatic ecosystems, with plants and phytoplankton facing decreased photosynthesis. Sediment can also clog fish gills. Up to a third of global freshwater fish species are found in tropical rivers, according to a 2021 study, but how many of those species are being impacted by mining isn’t fully known. Mining also increasingly threatens fisherfolk livelihoods, with local aquatic food supplies seriously curtailed.

Illegal gold mining in places like the Yanomami Indigenous Territory in the Brazilian Amazon has ravaged ecosystems and endangered residents: many Yanomami no longer have clean drinking water, children are poisoned by the mercury used to process gold ore, and criminal networks terrorize communities. Wherever river mining occurs, similar patterns occur.

Jacqueline Gerson, a biogeochemist who studies pollution due to gold mining at Michigan State University in the U.S., and was not involved in the current research, told Mongabay she thinks the new study makes a colossal research contribution. “By mapping out [global river mining], it sheds more light on the impact of all this compounding [harm]. It might be mostly small-scale activity, but it really does build up.”

This time series measures sediment in mining-impacted rivers across the tropics.
This time series measures sediment in mining-impacted rivers across the tropics. A clear trend toward intensified mining can be observed. The gap in the mid- to late 2000s in Brazil shows how responsive rivers can be to environmental policy. Image courtesy of Dethier et al. (2023).

Gold prices fuel mining boom

Gold accounts for 90% of tropical river mining today, making up 356 of the 396 mining sites studied. A third of these sites were established after 2009.

The current mining boom began not long after the 2008 stock market crash, when the price of gold took off. At the turn of the millennium, the U.K. sold 395 metric tons of gold at an average price of $275 per ounce. Today, an ounce is valued at $1,950; the price has never dipped below $1,200 in the last five years. Those prices provide a huge impetus to local people and to criminal syndicates to get into the mining game.

“Everywhere we looked, there seemed to be a change [to rivers] in the mid-2000s and early 2010s. We were seeing mining where there hadn’t been before,” Dethier said. “And suddenly, small mining operations were becoming enormous.”

Investors see gold as a financial safe haven against escalating geopolitical tensions (such as Russia’s ongoing war in Ukraine), a weakening dollar, and growing distrust in reckless financial markets — all trends that continue to intensify.

“Whenever everything else falls apart, gold goes up. And yet 20% to 25% of the world’s gold supply is mined by small-scale artisanal miners, in many cases, illegal or at least informal,” Luis Fernandez, the executive director of Wake Forest University’s Center for Amazonian Scientific Innovation (CINCIA), and one of the study’s co-authors, told Mongabay in a video interview.

In the Democratic Republic of Congo, 7 grams, or a quarter ounce, of gold now amounts to the average annual national wage. In Brazil, 3 g (0.1 oz) is more than a month’s minimum wage.

“If you are a construction worker in a capital city or a quinoa farmer in the Andes, when the price of gold is above $2,000 [an ounce], it’s a pretty good deal to leave your farm or maybe send your son to mine for a year or two, earning two orders of magnitude more than they earn being a farmer,” Fernandez said, “but [that activity] has had a really severe environmental toll.”

The good news is that rivers can recover relatively quickly if mining stops or slows.

“There are a few places like Brazil where we can see high sediment in the past, then it went away. Those correspond to policy interventions,” Silman said. “It is not an issue we don’t know how to solve.”

Banner image: Gold miners deforest riverine habitat and rip up riverbanks to extract tiny nuggets from soils. They then use the river to wash away the sediment, which can leave trails of muddy water downstream for thousands of kilometers. Image courtesy of Jason Houston/CINCIA.



Dethier, E. N., Silman, M., Leiva, J. D., Alqahtani, S., Fernandez, L. E., Pauca, P., … Lutz, D. A. (2023). A global rise in alluvial mining increases sediment load in tropical rivers. Nature, 620(7975), 787-793. doi:10.1038/s41586-023-06309-9

Blackman, R. C., Osathanunkul, M., Brantschen, J., Di Muri, C., Harper, L. R., Mächler, E., … Altermatt, F. (2021). Mapping biodiversity hotspots of fish communities in subtropical streams through environmental DNA. Scientific Reports, 11(1). doi:10.1038/s41598-021-89942-6

Vega, C. M., Orellana, J. D. Y., Oliveira, M. W., Hacon, S. S., & Basta, P. C. (2018). Human mercury exposure in Yanomami Indigenous villages from the Brazilian Amazon. International Journal of Environmental Research and Public Health, 15(6), 1051. doi:10.3390/ijerph15061051

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