We analyzed the impacts of deforestation on water quantity and quality in the Tapajós River, one of the main tributaries of the Amazon River. To that end, we used data from 2 years of fieldwork as well other state-of-the-art techniques in small and large areas of this study area. We found changes in water quality and quantity across small and large portions of this area. Our results show that some impacts of deforestation usually observed in small streams are also affecting the major river.