(currently over 800,000 people live within the river's catchment) has caused other problems in relation to sewage. Another major source of pollution until recently came from a disused tar distillery in Mirfield. In the middle 1950s the river was so polluted by dyeworks in the middle reaches between Todmorden and Sowerby Bridge, that it ran different opaque colours from day to day. On Easter Monday, after a long weekend holiday shutdown, the water cleared and the river bottom could actually be seen, looking like a grey wool blanket. Tighter controls during the 1950s led to an improvement in water quality, and presently organisations such as Calder Future are working collaboratively with local industries and Yorkshire Water to promote more responsible use of the river and to re-establish lost wildlife along its banks. Flooding and flood defences The river has a history of flooding, mainly due to the high sides of its banks in its earlier stages, which cause rapid runoff of water following heavy rain. Much of the lower part of the river has been urbanised, therefore trapping flowing water within the engineered river channels. Fast flows of water cause the deposition of sediment collected from the river banks, raising the river height further. A variety of flood defences are in operation along the Calder Valley to prevent the recurrence of floods which devastated communities in the early part of this century. At Wakefield, for example, the lake at Pugneys Country Park is used as an overflow for the river in order to protect the town. The recently regenerated Wakefield Waterfront area is defended by one of the most sophisticated flood defence systems in Europe. This complex of computer controlled automatic flood barriers are housed below ground and rise automatically when a flood threat is detected. Leisure use of the river The river is host to a variety of watersports activities: a canoeing centre at Sowerby Bridge waterskiing in gravel pits at Cromwell Bottom watersports and angling at Pugneys Country Park the use of leisure craft and narrowboats along the lower stretch of the river where navigation is made easier through the canals built during the Industrial Revolution Walking routes along the Pennine Way and Calderdale Way Angling in areas is distributed between the following; Bradford No.1 angling association, Mirfield AC, Thornhill AC, and Wakefield AC. Some stretches are free and only require a valid Environment agency rod licence See