University of Lancaster and the Dee Steering Committee to accurately predict when any release of pollutant would reach any of the main abstraction points on the river. The principle model flow data was provided from long-term data sets from Manley Hall gauging station which lies just upstream of the long, almost flat, serpentine section of the river path. The transit time through this stretch of the river can take several days in low-flow conditions. Additional data was obtained from gauges below the major reservoirs and from the sluices at Llyn Tegid. With the benefit of frequent data updates and with the addition of data from real incidents together with real-time data from Manley Hall, it has become possible to predict arrival times of pollutants at any one point downstream of Manley Hall within a window of a few minutes at low flows. Bala 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. Llyn Celyn 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 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