The Marañón is one of the last major free flowing tributaries to the Amazon. It is a vital link between the Andes Mountains and the Amazonian lowlands. Each year at high water the Marañón runs brown with sediment flowing down from the mountains. This is deposited in the rainforest, essentially feeding the Amazon with soil and nutrients that the ecosystems need to survive. If this connectivity between mountains and Amazon is broken, the results will be disastrous.