To overcome this problem, a series of reservoirs have been constructed to store the excess water available in the winter time and release it back into the River Dee during the drier months.
This was used by Thomas Telford at the beginning of the nineteenth century in order to guarantee a supply of water to the Ellesmere Canal.
Telford constructed sluices at the outlet of Bala Lake to control the flow downstream so that there was always sufficient water to supply the canal where it started at Horseshoe Falls.
This provided 18 million cubic metres of stored water in Bala Lake that could be controlled and used on a seasonal basis for low-flow regulation.
This enables continuous abstraction from the River Dee of 235,000 cubic metres per day by six statutory water undertakings and British Waterways Board.
This development was hugely controversial at the time and remains politically fraught to this day, since the construction of the reservoir involved flooding the Tryweryn Valley and the village of Capel Celyn and twelve farms.
To try to offset some of the environmental concerns associated with the scheme, some of the stored water was specifically set aside to make special releases to help fisheries, to provide recreational opportunities (canoeing and white-water rafting on the Afon Tryweryn) and to disperse pollution events should they occur.
This increased the potential for abstraction from the river in the lower reaches to nearly 900,000 cubic metres per day (200×10^6 imp gal/d).
The operating rules for the current system are agreed with all the participants and define the circumstances in which flow will be precisely managed.
[9] This location was chosen as it was on a section of the river where flow could be readily measured and above the very flat stretch that meanders into Cheshire.
This quality management system was developed following a severe Phenol pollution of the River Dee in the 1980s which resulted in contaminated water being supplied to large areas of Liverpool and the Wirral.
Although the statutory order was promoted by the Environment Agency it was supported and endorsed by the representative members of the Dee Regulation Scheme.