Reservoirs

The 39 reservoirs in the catchment are licensed to provide public supply and are operated by Yorkshire Water. A further four impounding reservoirs provide water for the canals within the Calder area. The catchment is virtually self-sufficient in water supply. Only small amounts of water are imported from outside the area, although the 1996 drought year was a notable exception.

Most of the water, which is for domestic and industrial use, is returned to the river via sewage treatment works. As a result the net outflow from the drainage basin is similar to that of the natural river. The flow is, however, highly modified by human usage.

Licences control the amount of daily flow from the reservoirs so that minimum flow in the river is maintained so as to protect downstream abstraction, other users and the quality of the water environment.