closure begins about 9 hours before a dangerous high tide reaches the barrier. messages give orders to stop river traffic, close subsidiary gates and alert other river users. as well as the thames barrier, the smaller gates along the thames tideway must be closed. once river navigation has been stopped and all gates closed, then the thames barrier itself can be closed. the smaller gates are closed first, then the main navigable spans in succession. the gates remain closed until the tide downstream of the barrier falls to the same level as the water level upstream.