Rivers integrate biological and chemical outputs of adjacent terrestrial ecosystems. With global temperature rise, Arctic ecosystems are changing dramatically, and with climate warming, more dissolved organic matter (DOM) inputs are received annually from the landscape. This reshapes the river’s metabolic balance, that is, the contribution of in-stream DOM degradation to carbon dioxide (CO2) outgassing. Here we strive to couple DOM flux and composition to river metabolism throughout the hydrological cycle of the River Tana (Norway) – the largest unregulated Scandinavian river. We installed in-situ sensors and collected samples across the watershed to cover the diversity of landscapes likely to generate a wide range of DOM. Our goal was to determine where the “hot spots” of DOM degradation are in space and time. Ongoing analyses revealed metabolic activities were 1) difficult to assess during the spring freshet, 2) peaked early in the season under high light and temperature, and 3) returned a substantial part of organic carbon fluxes back to the atmosphere. We identified the molecular transformations (reactive pathways) of DOM in the river from carbon stable isotope ratios, absorbance and fluorescence spectroscopy, and Fourier transform ion cyclotron resonance mass spectrometry. Terrestrial DOM signatures were conserved across the watershed, describing a dominant hydrologic connection in all parts of the catchment. DOM composition and reactivity were strikingly similar spatially and seasonally, yet more detailed analyses revealed distinct trends in heterogeneous content (CHO molecules containing N and/or S) and aromatic nature from upstream to downstream. Samples contained 1-10% biolability, common across alpine to outlet river continuums. We used a new conservative threshold for photolability and estimated that our samples contained 37-66% photoreactive compounds. Next steps are to represent DOM fluxes and transformations within a parsimonious process-based modelling framework. This will allow us to upscale to the whole catchment and predict changes in stream metabolism under climate change.