Reputation: 4577. One of the functions of the Locks on the Lake Washington Ship Canal (the probable little body of water you're referring to) is to keep the salt water and fresh water separate.Lake Union and Lake Washington are above sea-level (Puget Sound) so that's why the locks are necessary.eputation: 4577. One of the functions of the Locks on the Lake Washington Ship Canal (the probable little body of water you're referring to) is to keep the salt water and fresh water separate.