Geograph - photograph every grid square

River navigations developed gradually over centuries, and in the Middle Ages there was constant tension between millers, who erected weirs across rivers to channel flow through their millwheels, and the boatmen who found their passage impeded. (One clause in Magna Carta, apparently never enforced, calls for the removal of weirs from the rivers Thames and Medway.)
In an attempt to compromise between these interests, weirs were built with removable panels or, later, gates in them like a single lock gate. An opening would be created in the weir, allowing a surge of water through; a boat heading downstream would ride this surge through the weir, whilst one heading upstream would wait for the height of water on either side of the weir to approach a level and then be dragged through the gap; after this the gap would be closed and water would begin once again to build up behind the weir.
These navigable weirs were known as flash locks across much of the country, but as staunches in the east. They had various drawbacks. Most obviously, passage through for the boat was difficult and often dangerous, particularly downstream. They also made it harder for a navigable depth to be maintained: a boat travelling upstream would enter a length of river from which much water had just been drained off, and might find itself aground waiting for the water level to build up again. For this reason, as river navigations became more organised and increasingly run by joint-stock companies in the eighteenth and nineteenth centuries, flash locks were progressively replaced by more modern pound locks (those with an upper and lower pair of gates, between which the boat rises or falls as water is let into or out of the chamber), often on new cuts that completely bypassed the weir. Flash locks lingered, however, on less heavily-used navigations such as the upper Thames, and the last on that river, at Medley near Oxford, was not removed until 1928.