The Charles River watershed drains 308 square miles across twenty-three Massachusetts municipalities, yet its most photographed eight-mile stretch runs between the Museum of Science and Harvard's red-brick boathouses. Urban planners redirected the lower basin in 1910, replacing tidal mudflats with a controlled reservoir and a granite-edged esplanade that Frederick Law Olmsted had sketched decades earlier. The dam at Science Park holds the water at constant elevation, creating a mirror for the Back Bay skyline that shifts with every passing cloud. A Charles River boat tour Boston operators launch from CambridgeSide Place threads under nine bridges in sixty minutes, each crossing a chapter in civic infrastructure. The Longfellow Bridge's salt-and-pepper towers date to 1906, when streetcars clattered over granite arches. The sleek cable geometry of the Leonard P. Zakim Bridge, completed in 2003, carries ten lanes of Interstate 93 and anchors Boston's modern silhouette. Between them, the river widens past MIT's limestone dome and the Hatch Shell's concrete bandstand, where the Boston Pops play every Fourth of July. Narrated sightseeing cruises depart six days a week from the Cambridge shore, where the Galleria mall replaced a Lechmere trolley yard in 1990. Captains point out the Community Rowing boathouse, the Harvard crew docks, and the narrow channel where rowers practice at dawn. The basin holds seventy sailboats on summer weekends; in October, shells cut through fallen maple leaves that collect against the Esplanade seawall. Tour operators also offer Boston Harbor cruises that extend past the Charlestown Navy Yard and out to the Harbor Islands, and amphibious duck tours that begin on Boylston Street before splashing into the basin near the Museum of Science. The river's recreational identity is a twentieth-century invention. Before the dam, the lower Charles flooded twice daily, exposing tidal flats that smelled of raw sewage. The Metropolitan Park Commission began acquiring shoreline in 1893, but transforming the estuary into a public amenity required damming the mouth, dredging the channel, and building twenty-nine acres of fill. The project took seventeen years. By 1936, crews were sculling where clam diggers once worked at low tide.