River Don, South Yorkshire - Hydrology

Hydrology

The River Don, together with its main tributaries, the River Rother and the River Dearne, form a river system with a catchment of 714 square miles (1,850 km2), which held a population of around 1.4 million in 1997. Much of the region has an underlying geology of carboniferous rocks, containing coal measures, which have resulted in pollution of the river system where the coal has been mined. The headwaters rise on the moorlands of the Pennines, where the rocks are largely millstone grit, while the lower reaches pass through areas of alluvial and glacial material, up to 66 feet (20 m) think, which lies on top of strata of Magensian limestone and Sherwood sandstone.

The impacts of human habitation, particularly the inadequate treatment of sewage effluent, and the growth of mining in the upper reaches and the processing of metals in the Sheffield area have resulted in serious pollution of the river, and the depletion of fish stocks, to the extent that large parts of the Don contained no fish until the mid-1980s. Concerted efforts have been made to improve the water quality, by reducing the concentration of metals, the ammonia content, and the biological oxygen demand (BOD), which is a measure of the amount of oxygen that is needed by organisms to break down organic matter in the water. Sewage treatment works on the Don, notably those at Cheesebottom, on the west bank of the river at Thurgoland and Blackburn Meadows at Tinsley, have been upgraded, as has that at Darton on the River Dearne. This has led to a great improvement in ammonia levels and BOD, further aided by the construction of biological treatment facilities for the processing of industrial effluent in the Doncaster area, while the decline in the steel industry in Sheffield has reduced the discharge of metals into the river.

Mining presents a different problem, as the mines have been abandoned, but the pollution continues. Near Penistone, ochre discharges into the river from old ganister mine workings, while at Beeley Wood, the ochre comes from a pile of waste metal on the river bank. A nearby paper mill has also been a significant polluter of the river. Some of the problem has been mitigated by the construction of lagoons, into which mine discharges have been diverted. Water quality on the Dearne and the Rother has not improved as much as on the Don, and pollution of the lower reaches is compounded by the fact that the pollutants, which include dioxins, are locked up in the river bed sediments. Despite the steady improvement in water quality, restocking of the river with fish, attempted on several occasions between 1981 and 1994, was largely ineffective, caused by intermittent discharges of pollutants. In November 2011, the Environment Agency announced that they had recently re-stocked the Don with 1,000 barbel. A spokesman said that the fish in the river were now at a sustainable level with a breeding population and these would be the last fish added as part of a 10 year programme to help the Don recover from an industrial heritage that had depleted fish stocks.