Dams trigger long-term river-floodplain decoupling in dynamic Andean foreland rivers

Andean foreland rivers are preferential sites for ongoing and planned hydroelectric dam construction in the Amazon Basin. These rivers are characterized by high sediment fluxes that drive rapid rates of channel migration, large river-floodplain sediment exchanges, and creatediverse floodplain morphologies. Dams alter water and sediment supply to these dynamic rivers with expected morphodynamic and environmental impacts over decades to centuries. However, current observations and models of river response to dam construction do not fully incorporate the processes related to lateral channel migration and focus on short-term morphodynamic change.Here, we use an innovative hydro-morphodynamic model that accounts for flow, sediment transport, and channel-floodplain morphodynamics, including bend migration, dynamic vegetation, and channel abandonment. We demonstrate that Amazonian foreland rivers will undergo significant vertical and lateral erosion within less than 100 years following dam construction, with persistent or irreversible effects lasting for centuries. While downstream scour may partially offset the sediments trapped by dams, widespread erosion leads to formation of an entrenched inner secondary floodplain, which diminishes the extent and frequency of primary floodplain inundation. This entrenchment disconnects abandoned cutoff channels from the active river and potentially transforms floodplain ecosystems. Net erosion volume, and associated hydrologic changes, depend on pre-dam river characteristics: pre-dam rivers with high sediment loads and lateral migration rates experience the largest changes. Our simulations underscore the significant role of channel-floodplain interactions in driving fluvial reorganization and ecological adaptation downstream of dams. New dams could thus trigger a rapid and irreversible system reconfiguration, with critical impact on riparian ecosystems and the livelihoods of local communities.