Large rivers in forested landscapes transport substantial allochthonous organic carbon to estuaries and coasts. The East Texas Pineywoods is a coniferous forest region that drains to the Sabine-Neches Estuary, producing the largest annual discharge of freshwater to the Texas coast. We used a Fast Limnology Automated Measurement (FLAMe) platform to conduct a rapid spatial survey across >160 km of waterway including lower Sabine River, lower Neches River, coastal Sabine Lake, and Sabine Pass to the Gulf of Mexico. A prominent longitudinal gradient for dissolved organic carbon (DOC) concentration corresponded to a transition between visibly stained water from Sabine River and higher clarity ocean water, with mixtures of intermediate salinity and DOC in tidally influenced reaches. Longitudinal changes in DOC deviated from predictions of a conservative transport model based on salinity, indicating higher rates of DOC uptake or transformation at intermediate salinities. DOC transport and fate can greatly affect the underwater light environment. In waterways with a history of industrial pollution and contaminated sediment, such as Sabine-Neches Estuary, potential sensitivities of pollutant fate to benthic light or terrestrial-derived dissolved organic matter availability are not well understood.