MANAUS, Brazil — This year’s unusually severe fires in the Amazon have not only attracted widespread international attention, but also illuminated the effects of mounting deforestation in the region, from evaporating rains to rising carbon dioxide emissions. Yet one effect of forest loss in the Amazon has largely been ignored: how it influences the river system and the fish living in it.

There are few places in the world where aquatic and arboreal life are brought together as closely as they are in the Amazon. While the rainforest is home to the world’s largest river (by volume of water) and 1,700 tributaries, about one-sixth of the basin is also made up of largely forest-covered wetlands that flood for long