Step bedforms include a lot of incredibly diverse channel structures, which are widespread in different regions of the globe, including humid areas, desert ephemeral streams, semiarid environments, and alpine settings 10, 11. In high-gradient streams, where the granulometry is varied, riffles frequently give rise to sequences of steps in which local hydraulic discontinuities of the water flow are observed. Riffles represent a distinctive trait of most rivers worldwide 9. Consequently, quantifying gas emissions from upland freshwater systems is regarded as an important scientific challenge with multi-faced implications for a broad range of disciplines including ecology, biology, and climate sciences 5, 6, 7, 8. In particular, headwater streams are important greenhouse gas sources to the atmosphere, owing to the combination of enhanced input of dissolved matter from the surrounding landscape with high exchange rates across water-air interfaces. River networks transport, process, and release a multitude of chemical substances, which are relevant to the biogeochemical functioning of stream ecosystems and eventually affect the fragile interconnections of the land-water-climate nexus 1, 2, 3, 4. Therefore, global evasion from rivers may differ substantially from previously reported estimates. These findings suggest that steps are key missing components of existing scaling laws used for the assessment of gas fluxes across water-air interfaces. Our results indicate that local steps lead the reach-scale outgassing, especially for high and low discharges. The framework was applied to a representative, 1 km-long mountain reach in Italy, where carbon dioxide concentration drops across several steps and a reference segment without steps were measured under different hydrologic conditions. Here, we combine theoretical and experimental approaches to assess the contribution of localized steps to the gas evasion from river networks. Yet, capturing the imprint of the small-scale morphological complexity of channel forms on large-scale river outgassing represents a fundamental unresolved challenge. Steps are dominant morphologic traits of high-energy streams, where climatically- and biogeochemically-relevant gases are processed, transported to downstream ecosystems or released into the atmosphere.
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