Channelization of a large Alpine River: what it is left of its original morphodynamic?

The Adige River drains 12 200km2 of the Eastern Alps and flows for 213km within this mountain range. Similar to other large rivers in Central Europe, the Adige River was subject to massive channelization works during the 19th century. Thanks to the availability of several historical maps, this river represents a very valuable case study to document the extent to which the morphology of the river changed due to channelization and to understand how much is left of its original morphodynamics. The study was based on the analysis of seven sets of historical maps dating from 1803–1805 to 1915–1927, on geomorphological analysis, on the application of mathematical morphodynamic theories and on the application of bar and channel pattern prediction models. The study concerns 115 km of the main stem and 29km of its tributaries. In the pre-channelization conditions, the Adige River presented a prevalence of single-thread channel planforms. Multi-thread patterns developed only immediately downstream of the main confluences. During the 19th century, the Adige underwent considerable channel adjustment, consisting of channel narrowing, straightening, and reduction of bars and islands. Multi-thread and single-thread reaches evolved through different evolutionary trajectories, considering both the channel width and the bar/vegetation interaction. Bar and channel pattern predictors showed good correspondence with the observed patterns, including the development of multi-thread morphologies downstream of the confluences. Application of the free-bar predictor helped to interpret the strong reduction – almost complete loss – of exposed sediment bars after the channelization works, quantifying the riverbed inclination to form alternate bars. This morphological evolution can be observed in other Alpine rivers of similar size and similar massive channelization, therefore, a simplified conceptual model for large rivers subjected to channelization is proposed, showing that a relatively small difference in the engineered channel width may have a strong impact on the river dynamics, specifically on bar formation.