Dissolved organic matter (DOM) is viewed as an information-rich set of molecular tracers which carry the chemical signature of its source and subsequent journey to its point of analysis. The overarching goal of my research is to read the molecular signatures of DOM using advanced analytical techniques to reveal its source, reactivity, and environmental fate. Upstream landscapes and rainfall-runoff processes have emerged as major controllers of the quantity and quality of terrestrial DOM exported through inland waters to coastal margins. However, the mixing of fresh/marine waters and local biogeochemical cycles occurring within estuaries further alters the composition of terrestrial DOM which ultimately reaches the open ocean. My talk will focus upon hysteretic shifts in DOM composition during event-driven changes in river hydrology and variations in compound-specific stable carbon isotopic composition of dissolved black carbon, a fire-derived subcomponent of DOM, along an estuarine transect. I aim to build on these preliminary results to improve our understanding of the environmental controls on DOM composition and reactivity across hydrologic and estuarine gradients.