This structure was built in the late 19th Century as a barrier to retain water between here and Teddington at a half-tide level. Three of the arches of the bridge hold barriers that are closed during ther lower half of each tidal cycle. The fourth arch (on the right of the picture) holds a lock for vessels to use when ther barriers are closed. One usually makes tideway passages to or from Teddington at states of the tide when the barriers are open.