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Of Locks and Weirs
Locks and weirs are an integral part of the management of the Thames. Our weir, in particular, is often seen as a recreational asset, where canoeists can enjoy practicing their white-water skills. It is, in fact, a piece of industrial archaeology with a very long history. We are going to look at the origins of our lock and weir, and how they have evolved over the centuries to their present form.
The modern weir under repair in 2012
Most weirs are artificial and began as small dams, designed for excess water to spill over the top, which built up the pressure of the flow to turn a mill-wheel. The Romans introduced water-mills to England and, it seems, this was one innovation which the Saxons did not shun because, by the time of the Domesday book, six thousand of them were recorded.
Marlow had its mill in 1086, but whatever weir was here was not entirely artificial. This is the site of one of the five rapids on the Thames between Reading and London, formed by the end of the chalk ridge on which the High Street stands. Boats would originally have been ported overland round the weirs and rapids on wooden rollers. This was probably how the Vikings sailed a war fleet up to Reading. As trade on the Thames developed portage became impractical and another solution was necessary.
The medieval Thames was very different from the river we know now. It was shallow and very apt to overflow, flooding the water-meadows on either side. This could be useful for agriculture, but was often unhelpful. The Austin priors at Bisham complained that their buildings were constantly flooded and their animals suffered from foot-rot. The area immediately to the west of Marlow was covered by three large meres, known as the Aldermary Lakes. In a land survey of that area dating from 1658, at least ten pools, including these three, are listed. The holding of so much water within the land meant that the flow of the river was much reduced, making the construction of a mill dam very necessary.
This name, used for our weir for centuries, probably indicates its ancient beginnings. Rather than being just a dialect word, it is closely related to ‘Wehr’, the modern German word for ‘weir’. If the word is pronounced with an English W it sounds exactly as the title, so is probably Anglo-Saxon in derivation. The length of water to be dammed was daunting, so it is a tribute to medieval Marlovians that they managed the job so efficiently.
Weir and flash lock, pre 1872
This photograph, taken on the old ware some time before 1872, shows its impressive width. Follow the opening to the flash lock from the weir-pool, and you will see the paddle handles of the flash lock. On the right a Thames sailing barge is moored just below the church.
Probably, in the Saxon era, the dam only closed part of the space, raising the water level on the northern side sufficiently to turn the mill wheel. Victorian photographs, taken out on the dam, have recently come to light, giving us considerable insight into how it was constructed. Great blocks of chalk, reinforced below the dam by huge baulks of timber, probably of oak, form the outer skin of a dam at least twelve feet (four metres) wide, which was probably filled with packed soil and flint rubble. At the up-river end was an undammed section, allowing the river to fill the weir pool. Across the top of this was fixed a wooden frame which held a row of eel bucks. These baskets are recorded as being in use in 1086. The ones in Marlow remained until the early nineteenth century, while the rack at Harleyford still existed in 1900.
Eel bucks at Harleyford
A rack of eel bucks, still in place at Harleyford in 1900. A similar rack stood at the top end of Marlow ware until the early 19th century.
So large an obstruction to the river’s flow as the ware could cause problems when the Thames was in spate, so at some time two overflow channels were constructed. The upper one left the river in the area of Stoneyware, ran diagonally behind the Complete Angler and discharged into the weir pool. Its outlet is visible in a number of illustrations of old Marlow bridge, and the remains of the bridge over it can still be seen on the approach to the bridge on the Berkshire bank. The lower one was dug out of the mill wharves, now the gardens of the houses near the lock, and ran behind the mills, where it split into two. The right-hand branch discharged direct into the lower mill pool, the left-hand one ran across the ground below the mills on the Bucks bank to discharge just above the bypass bridge. The lower end of this was still visible in the mid-twentieth century as a dry dyke, at least two metres deep, which filled to the brim with water when the river was in flood.
Flood channel outlet on left
In this 1805 print the exit bridge of the top flood relief channel can be seen carrying the Bisham road behind the Compleat Angler, where the channel discharged into the weir-pool.