The Tideway is a part of the River Thames in England which is subject to tides. This stretch of water is downstream from Teddington Lock. The Tideway comprises the upper Thames Estuary including the Pool of London. Tidal activity Depending on the time of year, the river tide rises and falls twice a day by up to 7 m (24 ft). Because the tide goes against the outflow of fresh water from the Thames Basin, it takes longer to subside (6–9 hours) than it does to flow in (4–5 hours). London Bridge is used as the basis for published tide tables giving the times of high tide. High tide reaches Putney about 30 minutes later. Low-lying banks of London are naturally vulnerable to flooding by storm surges. The threat has increased due to a slow but continuous rise in high water level, caused by the extremely slow 'tilting' of Britain (up in the north and down in the south) due to post-glacial rebound and the gradual rise in sea levels due to climate change. The city and state have erected defensive barriers, including the Thames Barrier, which was constructed across the Thames at Woolwich to deal with this threat. Responsibilities The Tideway, often referred to as the Port of London, is managed by the Port of London Authority (PLA). The upstream limit of this authority is marked by an obelisk just short of Teddington Lock and to seaward by the London Stone at Yantlet Creek. The PLA is responsible for one lock on the Thames: Richmond Lock. Within Greater London, the Tideway is secured by the Metropolitan Police Marine Policing Unit. East of Crayford Ness, Essex Police and Kent Police assume responsibility in their respective jurisdictions. 21st-century criminal investigations have included the Roberto Calvi and Torso in the Thames cases. London Fire Brigade maintains a fire boat on the river in central London. As a result of the Marchioness disaster in 1989, when 51 people died, the Government asked the Maritime and Coastguard Agency, the Port of London Authority, and the Royal National Lifeboat Institution (RNLI) to work together to set up a dedicated Search and Rescue service for the tidal River Thames. Since 2002, four lifeboat stations have been established on the Thames, at: Teddington, Chiswick, Tower Pier, and Gravesend. Navigation The River is navigable to large ocean-going ships as far as the Pool of London at London Bridge.