The English River Thames is navigable from Cricklade (for very small, shallow boats) or Lechlade (for larger boats) to the sea, and this part of the river falls 71 meters (234 feet). There are 45 locks on the river, each with one or more adjacent weirs. These lock and weir combinations are used for controlling the flow of water down the river, most notably when there is a risk of flooding, and provide for navigation above the tideway. History From ancient times there were many obstructions across the Thames, for fish-pounds and millers' weirs. They are referred to by Asserius Menevensis in the ninth century and Magna Carta (1215) states that "weirs, for the time to come, shall be demolished in the Thames and Medway, except on the sea coast." It appears this never happened. In the Middle Ages, the fall on the river in its middle and upper sections was used to drive watermills for the production of flour and paper and various other purposes such as metal-beating. This involved the construction of weirs in order to divert water into the mills. The weirs, however, presented an obstacle to navigation and to solve this problem locks were built alongside the weirs to enable boats to be moved between levels. Originally these were flash locks that were essentially removable sections of weir. A boat moving downstream would wait above the lock until the lock was opened, which would allow a "flash" of water to pass through, carrying the boat with it. In the opposite direction boats would be winched or towed through the open lock. The difficulty of using flash locks, and the consequent loss of water and income to the miller, eventually led to their replacement with pound locks. Locks similar to these early pound locks still exist on the river, although in many cases they have been enlarged and mechanised. On the lower section, the river was tidal as far as Staines until the beginning of the 19th century and was under the control of the City of London. The City's jurisdiction was marked by the London Stone. The principle of lock/weir combination, which maintained the depth of water for navigation and reduced the danger of flooding, was extended over the tidal section as far as Teddington in a series of locks built after 1810. The first authority charged with managing navigation and lock building was the Oxford-Burcot