The Langevin River carves the steepest canyon system on Réunion Island, dropping 800 vertical metres from the Piton de la Fournaise watershed to Saint-Joseph in a succession of basalt gorges formed by successive lava flows between 150,000 and 5,000 years ago. The Grand Galet section — named for the polished boulders lodged in its narrows — became accessible to guided canyoning groups in 1998 after the first technical descent by local mountain guides mapped 14 rappels, five natural slides, and three mandatory jumps into emerald pools fed by year-round tropical runoff. The route threads through a hanging valley where endemic tree ferns and orchids cling to vertical walls, a botanical corridor protected under the Réunion National Park designation since 2007. Canyoning Langevin Grand Galet attracts 6,000 participants annually, making it the island's most frequented technical descent, yet the canyon's narrow geometry naturally limits groups to six clients per certified guide. The standard four-hour route begins at the Cascade de Grand Galet parking area at 450 metres elevation and exits at the lower river terrace, requiring eight abseils ranging from 8 to 25 metres over waterfalls that maintain flow even in Réunion's dry season between May and November. The longest rappel descends a tiered cascade where water volume triples during cyclonic swells, occasionally forcing closure of the route when the Météo-France alert system flags upstream precipitation above 50 millimetres per hour. Basalt friction here differs sharply from limestone or granite — the rock's micro-texture provides reliable purchase when dry but becomes glass-smooth under cascade spray, demanding precise anchor placement. The canyon's popularity stems from its accessibility to first-time participants — no prior climbing experience is required, though a comfort with cold water (year-round temperatures between 18 and 22 degrees Celsius) and a minimum fitness level to hike the one-kilometre approach trail are mandatory. Licensed operators supply full technical equipment including 5-millimetre neoprene wetsuits, helmets rated to EN 12492 standards, and figure-eight descenders, with harness adjustments made at the trailhead before entry. The route's middle section features three natural water slides where polished basalt forms chutes dropping into pools verified daily for depth and debris clearance, the longest slide measuring 12 metres with a mandatory seated entry technique taught on-site. Jumps of 3, 5, and 8 metres are sequenced to build confidence, each with a guide-tested landing zone marked by submerged reference rocks. The final abseil exits directly into the main Langevin riverbed, where the current slackens into a broad pool before continuing toward the coastal plain.