Engineering impacts on the Upper Rhine channel and floodplain over two centuries

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The reconstruction of long-term (>100 yr) channel changes is critical to understanding how natural and anthropogenic factors impact the evolution of river systems. It provides information on channel sensitivity and the limitations for channel repair in a functional river restoration context. This study presents a two-century analysis of the evolution of a large braided and anabranching river, the Upper Rhine downstream of Basel, which has experienced three phases of engineering works (nineteenth century channelization, 1930s groyne construction, and 1950s damming and by-passing). We studied historical maps, longitudinal profiles, and cross sections to investigate changes in the areas and geometries of fluvial features from 1828 to 2009. The results showed a drastic floodplain drying out following nineteenth century channelization (93% reduction of fully connected channels, 48% augmentation of partially connected channels, augmentation of disconnected water bodies by more than five times, and augmentation of terrestrialized channels of more than two times; 1828–1925). Changes propagated downstream at 280 m yr−1 on the 50-km long river continuum. The channel exhibited bed topography of large forms in 1880, with a median pool-riffle amplitude of 4.0 m, locally reaching 6.6 m. Among the three phases of engineering works on the Upper Rhine, channelization had the most impact on planform (floodplain terrestrialization) and vertical (7 cm yr−1 bed degradation) adjustments, with short reaction times and effects lasting several decades. Damming had the greatest impact on peak flows, and can explain the lower magnitude of the channel changes in the contemporary period. We synthetized the complex cause-effect relationships between natural/anthropogenic controlling factors and hydromorphological processes over the past 200 yr, and outlined the evolutionary trend of the main geomorphic variables, which can be used as a knowledge framework to evaluate future morphodynamic restoration schemes.