Kings Lock - the lowest manual lock on the river
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The Thames Locks
The locks keep the water deep, slow moving and navigable. Originally "flash" locks, merely removable sections in a weir that permitted a flash flood to carry a boat on down the river, pound locks started to be introduced in the 15th Century. The process of modernisation was finally completed in 1938! 
Narrow boats
The narrow boats in the picture are designed with a beam of 6' 10" to negotiate the rest of the inland waterways system, which interfaces with the Thames in the form of The Kennett and Avon, The Wey Navigation, The Basingstoke Canal, The Lee Navigation and most importantly The Grand Union, giving access to the rest of the system via Northampton. It is possible to travel (if slowly) east to the Wash and Humber, north to Lancaster and west to the Severn. 
The canals were built by the manual labour of the "navigators", later shortened to "navvies". With the introduction of railways the canals went into a gradual decline, now reversed by leisure use.