2015 large lock maintenance, Hiram M. Chittenden Locks, Seattle, Washington. In the western half of the main lock chamber, looking west at the lower gate of the large lock. Just behind the camera is the middle gate, which is regularly used to allow a use of the lock at half size; the full lock is used only for very long ships. Behind the lower gate, a special gate mitred in the opposite direction is used to seal the lock so that it can be pumped out for maintenance.