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

Ongoing and planned hydroelectric dam construction in the Andes will alter water and sediment delivery to downstream rivers, with likely substantial impacts on river and floodplain morphology, hydrology, and habitats. These disturbances will be greatest along laterally dynamic rivers with strong current channel-floodplain coupling, such as Andean foreland rivers. Using a hydro-morphodynamic river model, we show that these floodplains are rapidly transformed through widespread vertical and lateral erosion with persisting long-term effects: Erosion creates secondary floodplains entrenched several meters below pre-dam levels within a century. This degradation of the channel-floodplain complex changes floodplain inundation patterns and frequency and disconnects riparian water bodies from the active river. Such decoupling will be most severe in rivers characterized by high sediment loads and lateral migration rates before dam construction. Our simulations reveal that predictive approaches that neglect geomorphic changes, including lateral river mobility, may be of limited utility in assessing dam impacts on the construction and functioning of wetland habitats.