Answer to Question 4

The flow of the river in earlier centuries used to be much slower because there were far more obstructions to, and bottlenecks in, the flow of water. Bridges with narrow arches and wide piers impeded and slowed down the river flow. In order to render the river navigable, the river level has been permanently raised at numerous places along its course by the erection of weirs, with locks alongside to enable boats to pass from one stretch of river to another. Locks and weirs formed impediments to the passage of floodwaters as their sills (bases) were invariably above the natural bed of the river and they reduced the channel width. Mills were c6mmonly established at weirs to utilise the fall of water at these places, and since the miller wished to maintain a good head of water above his mill, he was reluctant to open the weir-sluices to their full extent, even at times of flood. Fish-traps, sometimes placed across the outlets of weirs, frequently became solid obstacles to the passage of water, owing to the weeds, leaves, branches, and floating rubbish which blocked the gaps in the wire netting.


It was not until the Thames Conservancy gained sufficient powers in 1894 to begin to manage the river Thames and its tributaries effectively that the flood problem began to improve. Locks, weirs and bridges have been replaced or removed, circuitous river bends have been straightened, and river channels have been widened and deepened by dredging. All these activities have contributed to a faster flow. The stretch of river in front of you is regularly dredged by the Rivers Division of Thames Water Authority, formed in 1974. A navigable depth of l.2 metres (4 feet) is maintained along this stretch of the river though the river's depth in places may reach twice this.