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FLASH LOCKS ON THE THAMES
Long before the introduction of modern pound locks, millers on the Thames built weirs to provide the head of water needed to operate their mills. Although the raised water levels helped boats to overcome shallows in the channel, through navigation was impossible without some kind of lock. Having a section of the weir that could be removed overcame the problem and 'Flash' locks as they became known were used for centuries until pound locks, with their two pairs of opening gates, were developed. The first pound lock to be used in England was built in 1564 by John Trew on the Exeter Ship Canal.
A flash lock would typically comprise a removable top beam laid across the opening section, with a combination of loose vertical 'paddles' and 'rymers' holding the water back, held in place by the water pressure. To open the flash lock a section of paddles was removed and the upper beam pulled back to open a channel for a boat to pass through.
Boats waiting at a flash lock to go down stream would be simply flushed through to the lower section, whilst those wanting to go up stream would wait until the flow had subsided and then be hauled through by the crew, helped by horses or a capstan. If operating a flash lock was a dangerous occupation for the keeper and boat crew spare a thought for the innocent traveller - when crossing what was normally a shallow ford - if the paddles were suddenly raised and the lock 'flashed'. It would be usual to sound a warning horn when the lock was about to 'flash', but even so not everybody made it across safely...!
When a flash lock was opened it drained the reach upstream, which could take a long time to refill. During this time the mill on the weir didn't have enough head of water to work, which did not please the mill owner. A compromise was reached between the milling and navigation users for the construction of canals running to the side of the main river, which were just wide and deep enough to take a barge, but with passing places. The narrow canal channel would fill up much faster than a reach of the main river and also waste less water during a 'flash', speeding up navigation, saving water and reducing milling down time.
To save water for the mills these canals could be allowed to drain when they were not actually being used for navigation and the damp bed of the canal grew lush riverside vegetation, making an ideal pasture for oxon and cattle. The settlement near North Maidenhead Cricket Club known as Ellington (originally "EAL LLAN TONE") is believed to have derived its name from this use, the old name literally meaning "The Ox enclosure of the permanent pasture"
A capstan from the last surviving Thames flash lock can still be seen today at Hurley, where it is maintained by SAS UK, the owners of the Whittington Estate. A paddle and rymer weir also remains in operation today at Northmoor in Oxfordshire.
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