Large gates closing. Second photo the gates are closed. Third photo is one of the hydraulic hinges ( really not sure what the technical term is ). Once the gates are closed the water level is either lowered or raised depending on which direction the boats are going. This is the northern lock that allows boats to move from Lake Washington out to Puget Sound. The water level has to be lowered because the water level of the lake is higher than that of Puget Sound.