To the casual observer, it can appear that the fresh waters inside the locks in Seattle merely had to be "connected" with a couple sections of canal to permit navigation between Puget Sound and Lake Washington. In reality, the operation was for more complex. The lakes were at differing levels, draining to a brackish tideflat where hundreds of boats and ships now moor.

Some details and some old photos describing the engineering challenges at

http://www.digital.turn-page.com/i/183009/3