River Thames Thames Estuary), the river is subject to tidal activity from the North Sea. Before the lock was installed, the river was tidal as far as Staines, about upstream. London, capital of Roman Britain, was established on two hills, now known as Cornhill and Ludgate Hill. These provided a firm base for a trading centre at the lowest possible point on the Thames. A river crossing was built at the site of London Bridge. London Bridge is now used as the basis for published tide tables giving the times of high tide. High tide reaches Putney about 30 minutes later than
HMS Queen Elizabeth (R08) "Gleaner" and 42 Regiment, Royal Engineers to gather information on the tides, the depth of the river bed, and the height of the three river crossings (Forth Bridge, Forth Road Bridge, and Queensferry Crossing). This was necessary as the most recent data available were 60 years old. "Queen Elizabeth" sailed on 26 June 2017 to undergo sea trials. The first stage of the operation was to move the ship from inside the fitting out basin, via one of the access gates, into the Firth of Forth itself, before taking her under the three Forth bridge crossings. Once this was accomplished,