“As the Native travelers illustrated, connecting saltwater and freshwater made sense, except for one large problem. Lake Washington was about 29 feet, and Lake Union about 20 feet, above sea level, while Salmon Bay was a tidal inlet with a water level that fluctuated 10 to 12 feet daily.” (HistoryLink.Org)
The Ship Canal opened in 1917 connecting the saltwater of Puget Sound to the freshwater of Lake Washington via Lake Union. It consists of two cuts, the Fremont Cut between Salmon Bay and Lake Union and the Montlake Cut between Lake Union and Lake Washington, and a set of locks at the west end of Salmon Bay. The Hiram M. Chittenden Locks accommodate approximately a 20-foot (6.1 m) difference in water level between Lake Washington and the sound. How did they do it?!
