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Biogeochemical processing of dissolved organic matter (DOM) in headwater rivers regulates aquatic food web dynamics, water quality, and carbon storage. Although headwater rivers are critical sources of energy to downstream ecosystems, underlying mechanisms structuring DOM composition and reactivity are not well quantified. By pairing mass spectrometry and fluorescence spectroscopy, here we show that hydrology and river geomorphology interactively shape molecular patterns in DOM composition. River segments with a single channel flowing across the valley bottom export DOM with a similar chemical profile through time. In contrast, segments with multiple channels of flow store large volumes of water during peak flows, which they release downstream throughout the summer. As flows subside, losses of lateral floodplain connectivity significantly increase the heterogeneity of DOM exported downstream. By linking geomorphologic landscape-scale processes with microbial metabolism, we show DOM heterogeneity increases as a function of fluvial complexity, with implications for ecosystem function and watershed management.

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Rivers play a significant role in terrestrial carbon budgets through release of organic carbon (OC) to the atmosphere, delivery to oceans (Aufdenkampe et al., 2011;Galy et al., 2015), and storage in downed, dead wood, and floodplain soil (Sutfin et al., 2016). Combined OC storage in the atmosphere and biosphere is less than storage in soils (Falkowski et al., 2000), which have the potential to retain OC at depth along river corridors (