Agricultural and urban land use intensifies riverine GHG emissions across continents.

Earlier efforts to assess anthropogenic impacts on river greenhouse gas (GHG) emissions mainly relied on local-scale data, overlooking how cross-boundary factors such as climate, topography, socio-economic and demographic context shape river pollution and GHG emissions. To better understand the influence of urbanization and agriculture on riverine GHG fluxes, we measured fluxes at 156 sites across four river basins and continents. We linked land use and demographic contexts to river biogeochemistry via the human footprint methodology and explainable machine learning. Rivers crossing densely populated areas became CH4 hotspots when waste generation outpaced treatment capacity, as untreated effluents and flow-controlled urban channels, increased residence times and promoted anoxic conditions conducive to methanogenesis. These rivers emitted two times more than other land use classes and up to 200 times more than urban sites with adequate infrastructure. Cropland sites exhibited the highest CO2 and N2O fluxes, which were double those from urban sites, driven by agricultural practices promoting lateral transport delivering both substrates (nutrients and organic matter) and dissolved GHGs to rivers. These results demonstrate that pollution from human activities, rather than river size or Strahler order, is the dominant control on river GHG fluxes. Accordingly, improving land and wastewater management to reduce pollutants entering rivers could significantly mitigate elevated riverine GHG emissions.