The water demands of North West England including Liverpool and the Wirral far exceed the locally available sources of clean water. The River Dee runs mainly in North Wales before flowing through Chester, England, and then returning to Wales in a man-made channel constructed to gain land from the Dee Estuary. The Dee is the largest relatively clean river near to the North West conurbation and without water from the Dee much of Liverpool would be without water. However, the natural flow of the River Dee during most summers is insufficient to sustain any significant abstractions. 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 is the principle of low-flow regulation. This was used by Thomas Telfordat 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. The River Dee has also been used for direct drinking water supply with the Alwen Reservoir, built in the 1920s to supply Birkenhead with water. In the industrial revolution many rivers in industrial areas became too polluted by effluents to be usable for water supply. The Dee however remained clean with relatively few polluting effluents in the Dee catchment upstream of Chester. Consequently, the City of Chester has been able to directly abstract Dee water since the first Chester Waterworks Company was formed in 1826.
In the late 1950s the Bala Lake Scheme was promoted to increase the available water for abstraction in the River Dee. Telford's original sluices were by-passed and the natural lake outlet was lowered. New sluice gates were constructed downstream of the confluence with the Afon Tryweryn, which is only a short distance from the lake exit. 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. An additional benefit was a reduction in flooding events downstream of Bala as Bala Lake was able to contain the worst of the winter flood peaks.
As demand for water increased it was necessary to increase the storage on the River Dee and the next development was Llyn Celyn, a new 81,000,000-cubic-metre capacity regulating reservoir within the Bala Lake catchment area. This was completed in 1967 by Liverpool Corporation, and designed to operate in conjunction with the Bala Lake Scheme. This enables additional Dee abstractions of 327,000 cubic metres litres per day together with additional flood control storage. In summer time the impact was to increase threefold the dry-weather flow for most of the length of the river. 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. Local people saw this a destruction of part of the Welsh culture in order to supply England with water. This caused a great deal of controversy, resentment and protest. 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 and to disperse pollution events should they occur. A four megawatt hydro-electric station at the dam was also included in the scheme.
Further statutory powers were gained in 1973 to construct another major regulating reservoir in the River Brenig valley - Llyn Brenig. This reservoir was first filled in 1979 providing an additional of storage. This increased the potential for abstraction from the river in the lower reaches to nearly. By 2002 the authorised abstractions had been taken over by three statutory undertakings and British Waterways Board with a total licensed abstraction of . In addition a residual flow of at least 364,000 cubic metres per day is maintained over Chester Weir in all but the most testing of droughts, safeguarding the passage of migratory fish and limiting the ingress of saline water over Chester Weir during high tides.
Operating rules
The operating rules for the current system are agreed with all the participants and define the circumstances in which flow will be precisely managed. The key measuring location is at Manley Hall, a gauging station near Chirk. 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. The current rules state that when flow exceeds 10 m³/s no intervention is required. In practice some adjustment of Bala sluices may take place to increase storage in Llyn Celyn and conversely some releases may be made from Llyn Celyn for recreational or power generation purposes. When the flow at Manley Hall decreases towards 10 m³/s, additional flow is released from Llyn Tegid. If that is insufficient, flow from Llyn Celyn is used to maintain 10 m³/s at Manley Hall. In extreme situations where the flow from Llyn Celyn is insufficient to maintain the flow, releases of water from Llyn Brenig are made. Circumstances can arise where even this is insufficient to maintain flows and in such cases drought provisions are agreed which progressively reduce the maintained flow at Manley Hall. The overall effect of this regulation has a marked impact on the hydrograph of the River Dee. In dry years the hydrograph flat-lines at 10 m³/s whilst dry weather continues such as in 1990
Quality management
The River Dee regulation system also manages a water quality monitoring and alerting system that includes real-time quality monitoring for a wide range of chemical parameters supplemented by daily fixed site monitoring with analysis provided in near real-time by a dedicated laboratory service. The results of the analysis are made available to the four participating organisations on a routine daily basis. For each of the critical water quality parameters alert levels and action levels have been set based on past experience. If an alert level is exceeded an immediate alert is issued to all participants. As contamination levels increase or the contaminant plug nears an abstraction point the DEEPOL level rises to DEEPOL 2 and finally DEEPOL 3 at which time the affected abstractions are closed until the river quality returns to normal. 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. Contamination of raw water is a more significant problem on the River Dee because the River Dee is normally of exceptionally good quality and as a result the abstractions have been constructed directly from the river rather than via bank-side storage reservoirs as is more common when taking water from Rivers of less reliable quality.
