River systems play a pivotal role in transporting and transforming organic carbon (OC) fixed by terrestrial primary production. However, there is a fundamental gap in our understanding of the connectivity of terrestrial, aquatic, and marine carbon budgets due to a lack of measurements along the lower (i.e. tidally-influenced) reaches of large river systems. For example, all estimates of carbon fluxes from the world's largest river, the Amazon, are based on measurements made at and upstream of Óbidos, roughly 900 km from the mouth. Here we examine the evolution of OC concentrations and composition from Óbidos to two discreet channels near the mouth of the Amazon River during five cruises from 2010 to 2012. OC characteristics of the Tapajós River, which enters the Amazon River downstream of Óbidos, and the Tocantins River, which mixes with the Amazon River plume in the Atlantic Ocean, were also assessed.