The outflow of Leverett Glacier, a large land-terminating glacier of the Greenland Ice Sheet was sampled over the 2015 ablation season. Dissolved organic carbon (DOC) concentrations, dissolved organic matter (DOM) fluorescence, and hydrogeochemical data (e.g. discharge, specific conductivity, pH, and turbidity) were analyzed to assess changing DOM sources over the melt season. DOC concentrations and red-shifted fluorescence suggest terrestrial inputs from overridden soils dominated DOM early season inputs before progressive dilution with increasing discharge. During the outburst period, supraglacial drainage events disrupted the subglacial drainage system and introduced dominant protein-like fluorescence signatures not observed in basal flow. These results suggest that subglacial hydrology and changing water sources influence exported DOC concentration and DOM composition, differentiated through fluorescence characteristics. The outburst and post-outburst periods were characterized by protein-like fluorescence from supraglacial and potentially subglacial microbial sources.