The Dee regulation scheme is a system of flow balancing and quality management along the River Dee managed by a consortium of the three largest water companies licensed to take water from the river, United Utilities, Welsh Water and Severn Trent Water; together with the regulator, Natural Resources Wales. Water demand 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 left near to the North West conurbation and without water from the Dee much of Liverpool would be without water, unless local rivers such as the Mersey were rehabilitated. 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 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. 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. A project to review the pressures and opportunities for the Dee abstractions was undertaken and published in 1996. One of the outcomes of this report was the development and implementation of the River Dee Water Protection Zone. Flow model In order to better manage flow and the substantial abstractions of water from the river, a mathematical model was developed by the