There are landscapes that deceive. Walking along the Rhine in Alsace, it might seem as though you are admiring a powerful, continuous river as it has always been. However, what most hikers take for the Rhine is often only a greatly reduced version of the original. The true giant, which once carried the majority of the flow, now flows through a man-made canal. The Grand Canal of Alsace, built between 1932 and 1959 over about 50 kilometers, includes four drops equipped with hydroelectric power plants and locks at Kembs, Ottmarsheim, Fessenheim, and Vogelgrun. This project created the Rhine Island, a 51-kilometer-long strip of land, classified in 1967 and later designated a federal hunting reserve in 1971. By diverting the majority of the flow, the project dried up the historical bed of the Rhine and caused the disappearance of many wetlands, floodplain forests, and surrounding inundated meadows. Before the major developments of the 20th century, the Alsatian Rhine was not the long, calm river it appears to be today. It was a capricious watercourse, with multiple secondary channels, shifting gravel bars, and devastating floods that redrew the landscape every season. Navigating it was almost a gamble, as water levels and navigable channels constantly changed. This chronic instability was not a novelty of the 20th century: as early as the 19th century, the Baden engineer Johann Gottfried Tulla had already tackled the problem by straightening the river's course, which had the effect of drying up the first set of secondary channels. But after the 1920s, needs had changed in scale. France wanted a reliable navigable axis to develop river trade, while also seeking to exploit the considerable energy potential of the Rhine. The wild river, with its caprices and unpredictability, had become an economic obstacle as much as a danger to the surrounding residents. Therefore, it was no longer a matter of taming the Rhine in its original bed, but literally building a second artificial one, more docile and more useful. Fifty kilometers of concrete to tame a giant. Thus was born the Grand Canal of Alsace, a titanic project spread between 1932, the year of completion of the Kembs drop, and 1959, the date of commissioning of the Vogelgrun drop, with the entire project fully completed in 1961. Over about 50 kilometers, dug parallel to the Rhine on the French side, the engineers shaped an artificial watercourse capable of absorbing the majority of the river's flow. The development is not limited to a simple giant ditch. It includes four water drops equipped with hydroelectric power plants, installed at Kembs, Ottmarsheim, Fessenheim, and Vogelgrun, each associated with double locks allowing barges to cross these artificial elevation changes. The canal, now operated by EDF, guarantees a minimum navigation depth throughout the year, with an average level approaching 10 meters at certain places. A technical feat that transformed a section of the Rhine into a true river highway, while producing precious electricity for the region. This project also created an unexpected geographical object: the Rhine Island, a 51-kilometer strip of land trapped between the Old Rhine and the new canal. For a long time, it remained outside of any intensive exploitation, being classified in 1967 and later becoming a federal hunting reserve in 1971, a status that allowed the preservation of part of its biodiversity despite the surrounding changes. The historical bed of the river, the forgotten one of the century's project. Here is the central paradox of this story: by diverting the majority of the flow to the new canal, the engineers left the historical bed of the Rhine in a residual state, almost symbolic. The original river, which had shaped the valley for millennia, found itself reduced to a trickle compared to its former flow. Some sectors illustrate this transformation particularly well, such as the Fessenheim channel, dug as close as possible to the ancient course of the Rhine, directly in its historical bed, a technique that allowed limiting the project's impact on the surrounding agricultural land, but which further intensified the local drying. The ecological consequences of this hydraulic shift have been profound. The almost complete disappearance of wetlands around the river has disrupted entire ecosystems, accustomed for centuries to the natural fluctuations of the Rhine. The floodplain forests, reed beds, and inundated meadows, once nourished by regular floods, found themselves deprived of their main source of life. A silent, invisible change for most residents, but very real for naturalists who observed these changes on the ground over the decades. What the Grand Canal of Alsace has changed forever. The story does not end with the disappearance of wetlands. On the Rhine Island, the large areas cleared during the project experienced an extremely slow re-vegetation, giving rise to dry and original habitats that eventually fascinated naturalists in the 1980s. This ecological paradox summarizes the complexity of the subject well: a destructive project in the short term ended up, in the long term, creating unique habitats, almost unique in Alsace. This contrast between the initial objective of the project, namely modernizing navigation, producing electricity, and protecting the surrounding population from floods, and its real ecological consequences continues to be documented by local conservation actors in Alsace. The Grand Canal of Alsace thus remains an ambivalent symbol: that of an undeniable engineering success, but also a constant reminder that every landscape transformation has a cost, sometimes paid long after the bulldozers have left the site. Today, walking on the Rhine Island or along the almost dormant Old Rhine, one can measure how much the Alsatian landscape bears the traces of this choice made in the last century. Between technical prowess and ecological scar, the Grand Canal of Alsace invites us to rethink what it really means to "modernize" a river. And if the best lesson from this story lay precisely in this question: how far are we willing to transform nature to make it more useful, without ever being able to go back?