Engineered Continental‐Scale Rivers Can Drive Changes in the Carbon Cycle

Abstract Floodplains of large alluvial rivers modulate the composition riverine organic carbon (OC) and control OC oxidative loss, constituting a critical component in global river‐atmosphere‐ocean cycle. Therefore, anthropogenic management disconnecting from their floodplains is expected to reduce loss change quality quantity exported ocean. Here, we test this idea by combining two chronometers— 14 C age spectra optically stimulated luminescence ages quartz—to interrogate sediments Lower Mississippi River (LMR) system constrain effects on cycling continental‐scale sediment routing system. The LMR has been reduced >5,000 yr prehistoric <3,000 historic modern with significantly narrowed spectrum width, following centuries embanking LMR. Bank stabilization river‐floodplain exchange ∼90%, effectively cutting off older floodplain river reducing residence time severely truncated system, expedited downstream transmission OC. will have decreased enhanced younger delivery marine sediments. We estimate that ≥ 1.1 Tg C/yr or 40%. Extrapolation other undergone changes similar illustrates process likely represents sink can increase if currently free‐flowing tropical are embanked future.