Though Johnston was not a part of the research team, he is familiar with the Amazon mapping effort. He participated in related research in 2000, which was funded in part by the National Geographic Society. (National Geographic News is part of the National Geographic Society.)
"The mouth of the Amazon is so wide, it's hard to pick the exact spot at which to choose the end point with that kind of precision," he said.
"Personally, I would want to know a little bit more about how they came to that number before I was comfortable saying, 'Yes this is longer,'" he added.
Glenn Switkes is the São Paulo-based Latin America coordinator for the conservation group International Rivers Network.
The debate over river length is trivial, he said, but he hopes the announcement will focus international attention on the Brazilian government's efforts to build more than 60 large dams on the Amazon's major tributaries.
"The Amazon continues to be by far the river system which sustains the most biodiversity and which continues to maintain a natural state throughout most of its course," he said.
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