Dedicated on July 4, 1917, the Hiram M. Chittenden Locks was the largest navigation facility in North America. During the dedication, the USS Roosevelt, flagship for Commodore Robert Peary's famous 1909 expedition to the North Pole, led a parade of boats through the locks. The locks control the elevation of the lakes behind the Hiram M. Chittenden Locks, allowing vessels to move back and forth from one water elevation to another. At the locks, the freshwater of Salmon Bay meets the saltwater of Puget Sound, and depending on tides there can be a 6-25 foot difference between fresh- and saltwater. Using chambers, gates, tunnels, and valves, the lockmaster in the control tower opens and closes the large lock so vessels can enter, be enclosed in a chamber, and have water flow in and out through tunnels. Water moves by gravity from a higher to lower level, with no pumps used, and the average lockage takes about 7.5 million gallons of freshwater to fill the large lock.