It’s been debated for centuries where the Amazon River officially starts and scientists now believe they’ve found the river’s true source.

The authors of a study published in Area  say Peru’s southwestern Mantaro River is the true source, replacing the Apurimac River that has been attributed as the source since 1971.

Using topographic maps, GPS tracking data and satellite imagery—professional kayaker James Contos and his team determined that the Mantaro is about 10 percent longer than the Apurímac River. If the claim were true, the Amazon would add 47 to 57  miles  (75 to 92 km) onto its already mighty 4,000-mile (6,437 km) length.

It’s curious to think the source world’s largest river- by volume- has been so tricky to pin down but at least five Peruvian rivers have grabbed the title at some point since the mid-1600s. The question is complicated by the number of tributaries that feed into the mighty river making the crowning of a true source often a game of linguistic jargon.

The definition of “source” isn’t entirely established and whether geographers accept the claim of the Mantaro as the Amazon’s origin depends on which definition is applied.

National Geographic’s currently accepted definition is “the most distant point of a river’s longest tributary that flows continuously” but that still leaves significant room for debate.

One of those debates is how can the Mantaro be the true source if it doesn’t flow year-round? The Tablachaca Dam constructed in 1974 causes the river to run dry for nearly five month by diverting its flow around a kink, leaving the loop devoid of water. The question of man-made alterations to an environment is at the heart of the great Amazon River debate.

In their study, Contos and Tripcevich argue that the length of a tributary should trump whether it flows year-round. One member of the expedition team, Geographer with the Smithsonian’s National Air and Space Museum Andrew Johnston believes “the Mantaro could be considered a new source of the Amazon,” but not “the source.”

When water is flowing through the Mantaro, it “probably has a greater flow distance than any other Amazon tributary,” Johnston says. But when the Mantaro is dry, the formerly accepted true source Apurímac is once again where water flows the greatest distance into the Amazon.

Regardless of the Amazon River’s origin, the river plays a critical role in supporting the world’s largest tropical rainforest as well as regulating global climate stability. As the source of one-fifth of all fresh water on the planet, the Amazon Basin drives global atmospheric circulation, affecting precipitation across the Americas. It is crucial to protect the Amazon River as well as the forests surrounding it to ensure a healthy global climate for all.