The Canal Saint-Martin opened in 1825 to deliver potable water from the Ourcq basin to 100,000 Parisians, but its four-kilometre route through the 10th arrondissement became something more valuable: a pedestrian corridor lined with chestnut trees, nine double locks, and two swing bridges that halt traffic every time a barge passes. Napoleon ordered its construction in 1802 after a drought left the city's fountains dry. The canal drops twenty-five metres from the Bassin de la Villette to the Port de l'Arsenal, where it meets the Seine near the Bastille, and that gradient is absorbed by the iron gates and granite sluices that still operate on their original 19th-century mechanism. A canal Saint-Martin and Seine river cruise stitches together two incompatible hydraulic systems: the narrow, lockbound canal and the tidal river. The boats that make this journey are flat-hulled barges narrow enough to pass beneath the arched footbridges at rue Dieu and rue Bichat, yet stable enough to join the Seine at the Port de l'Arsenal and continue downstream past Notre-Dame and the Louvre. Most waterway tours in Paris stay on the Seine, never entering the canal, which remains a working transport route used by maintenance vessels and commercial barges. The transition between systems happens at the Arsenal basin, where the canal's still water meets the river's westward current. The canal's towpaths were once trodden by horses pulling freight barges loaded with grain, wine, and Burgundy stone. By 1860, the Haussmann renovations buried a portion of the canal beneath Boulevard Richard-Lenoir, creating the subterranean vault that boats enter near République and emerge from at the Bastille. Above ground, the canal became a backdrop for Marcel Carné's 1938 film Hôtel du Nord, which fixed its iron footbridges and working-class quays in the city's visual memory. Today the towpaths are unbroken promenades, the warehouses converted to studios and bookshops, but the locks still flood and drain with the same hand-cranked wheels. The Seine portion of the route covers the monumental axis: the glass pyramid of the Louvre, the Gothic buttresses of Notre-Dame, the Pont Neuf with its 384 stone masks, and the iron ribs of the Pont Alexandre III. These are static landmarks, but the canal is a moving system, a piece of civil engineering that visitors pass through rather than observe from a distance. The lockkeepers still live in the gatehouses at each sluice, operating the machinery by hand, and the rhythm of the journey is determined not by a schedule but by the canal's hydraulic logic.