Severe drought reveals more than 100 rock carvings in Amazonian tributary that may be up to 2,000 years old

A record drought hitting the Amazon rainforest has revealed petroglyphs carved by humans up to 2,000 years ago that are now partly submerged in one of the largest rivers in the region. The pre-Columbian engravings were found at a point in the Brazilian city of Manaus, where the Negro River's dark waters meet the murky current of the Amazon River.

The images carved into the rocks depict humans, animals and geometric shapes, such as squares and circles, according to Indigenous archaeologist Carlos Augusto da Silva, from the Federal University of Amazonas, who is studying the petroglyphs.

Archaeologists have caught glimpses of the enigmatic carvings before — in 2005, 2009 and 2010 — when severe droughts hit the region and the river's water level dropped, Silva told Live Science in Portuguese. The petroglyphs were recorded officially in 2010 by Brazil's National Historical and Artistic Heritage Institute (IPHAN), Silva said.

In an expedition on Oct. 21, Silva counted more than 100 petroglyphs, the most he has ever identified on the Negro River.

The petroglyphs are carved into rocks at "Encontro das Águas" (Portuguese for "Meeting of the Waters"), which is a Brazilian Cultural Heritage site due to its scenic value and the ethnographic and archaeological findings that have been discovered there — including artifacts buried near the riverbank that have been found there since the 1970s. The latest findings of the underwater petroglyphs appeared at the beginning of October after more than half the height of the river dried up.

Related: 800-year-old spiral rock carvings marked the solstices for Native Americans

This year, the Negro River reached its lowest point since records began in 1902, according to the Port of Manaus, located about 8 miles (13 kilometers) away from the petroglyphs. And the level is continuing to drop: In October, the river level is dropping an average of 4.2 inches (10.6 centimeters) daily.

The Brazilian Ministry of the Environment announced that the drought in the Amazon rainforest is the result of a combination of factors, noting "the El Niño phenomenon is worsened by climate change, plus there is dried out organic matter in large quantities, and criminal fire on private and public areas."

 Water levels in the Amazon have fluctuated over the millennia, for instance when the climate was drier between 8,000 to 4,000 years ago and the river level was likely much lower than it is today, Eduardo Góes Neves, an archaeologist at the University of São Paulo who was not directly involved in the finding, told Live Science in Portuguese. However, the current drought is linked, in part, to human-caused factors, rather than purely natural causes. He noted that this month's sighting may be the longest period the petroglyphs will stay exposed above water in modern times because the region is so parched.

How old are the petroglyphs?

Archaeologists aren't sure exactly when the petroglyphs were carved. "It is difficult to date excavations directly in the rock if there is no organic material associated with it," Neves said.

But the area holds clues about their age. Ceramics found in the archaeological site at the top of the Negro River's riverbank near the petroglyphs date to 2,000 years ago, according to radiocarbon dating. "It's [too] early to say that the same population made the carvings," Silva said.

Neves noted that Encontro das Águas is very dynamic, and the ceramics could have been transported there by the rivers, meaning the people who made the ceramics might be different to those who crafted the petroglyphs. In addition, both archaeologists said similar engravings have appeared in many places along Amazonian rivers, including around 500 miles northwest (800 km) from Manaus.

Usually, the water helps hide and preserve the petroglyphs, so their current exposure concerns archaeologists — especially since curiosity is drawing people to the location. Brazilian law prohibits the damage of archaeological artifacts, prompting IPHAN to engage the public with supervised educational activities aimed at environmental and heritage preservation in the coming days, according to a translated statement.

For Silva, the findings highlight the intelligence and skill of the area's past inhabitants. "They are a sort of rebirth of the Amazon in history, once brutally denied by Western society, which in literature considered the original peoples as intellectually disabled," he said. "Carving a design in a rocky material that lasts a long time requires a technique that is pure pedagogy, knowledge passed for generations."