Near Saint-Malo, the Rance is no longer a typical river. Since the mid-1960s, a dam has sealed off its estuary, converting the natural rise and fall of tides into a continuous source of electricity. This system uses turbines that spin both when the sea rises and when it recedes, capturing energy in both directions. This innovation made the Rance tidal power plant a global pioneer, still operating nearly six decades later. However, despite its success, no other large-scale tidal power plants have been built in France or elsewhere, a fact that raises questions about the balance between technological progress and environmental impact. The construction of the plant required a dramatic and unusual approach. To build a structure that harnesses the tide, engineers needed to work in a dry environment, something impossible in a constantly flowing estuary. To achieve this, they built two temporary dikes on either side of the site, isolating the area from the sea. This feat relied on a special technique developed by engineer Albert Caquot, using large concrete caissons and sheet pile gabions to create a dry construction zone. From 1963 to 1966, the Rance was completely cut off from the ocean, transforming the once-dynamic estuary into a still body of water. Construction began in July 1963 and was completed in 1966, with President Charles de Gaulle inaugurating the plant on November 26, 1966. At the time, environmental considerations were virtually nonexistent. There were no impact studies or regulations to assess the effects of isolating the estuary, which had immediate consequences on its ecosystem. The drastic change disrupted the natural flow of water and sediment, leading to long-term environmental changes. The dam has contributed to the gradual buildup of silt in the estuary, altering the landscape and affecting local wildlife. White sand beaches have been replaced by mudflats, and species like the sea lamprey and eel have disappeared, while faster-moving fish such as sea bass and mullet have adapted to the new conditions. Despite its pioneering status, the Rance plant has never been replicated in France, and only a few similar projects exist globally. The reasons are complex. Suitable locations for such projects are rare, requiring both a large tidal range and a physically closable estuary. Additionally, the high cost of these structures—often taking decades to amortize—does not align with current investment trends. Perhaps most importantly, the environmental consequences observed at Rance have acted as a deterrent. The British Severn estuary, once considered for a similar project, was abandoned for the same ecological reasons. Today, only four tidal power plants operate worldwide, with the Rance accounting for nearly 90% of global tidal energy production. While it remains a technological marvel, its story highlights the delicate balance between harnessing natural forces and preserving the ecosystems that sustain them.