The construction of Boston’s Charles River Dam was a monumental project that transformed the tidal estuary of the Back Bay into a fresh-water basin, providing a 20th century solution to problems that the city inherited from the 19th, including issues with industrial waste, sanitation, and general public distaste for the acres of mudflats that were exposed at low tide. Temporary floodgates closed on October 20, 1908, which marked the first separation of the waters of Boston Harbor from the Back Bay’s brackish salt marsh. In the lead-up to this moment, earthen dams were constructed on both sides of the river, with a lock allowing boats to pass through the dam on the Boston side and a sluiceway to regulate water levels in the upstream basin on the Cambridge side. A temporary wooden dam was built to close the center of the river, allowing for the construction of a permanent dam made of dirt and rock. Despite facing opposition and challenges, the dam was successfully completed in 1910, resulting in the creation of the Charles River Basin, the Esplanade, and some of Boston’s most iconic sites for outdoor recreation.