The Calder rises on the green eastern slopes of the Pennines flows through alternating green countryside, former woollen-mill villages, and large and small towns before joining the River Aire near Castleford.
The river's valley is known as Calderdale, which gives its name to the large urban and rural borough (centred on Halifax) through which the upper river flows. The lower reaches flow through the boroughs of Kirklees (based on Huddersfield) and Wakefield. However, the river does not flow through the centres of Halifax and Huddersfield, which are on the Calder's main tributaries, the River Hebble and River Colne respectively. The only large town centres through which the Calder flows are Brighouse,Mirfield, Dewsbury and the city of Wakefield.
The river itself is only navigable in short sections, but these sections are connected by artificial "cuts" (eg Horbury Cut) to form the Calder and Hebble Navigation, a popular leisure waterway which is part of the connected inland waterway network of England and Wales.
The river rises approximately 400 m above sea level at Heald Moor, north-west of Todmorden, and drains an area of 957 km². It flows for a distance of around 72 km / 45 miles through Todmorden, Eastwood, Hebden Bridge, Mytholmroyd, Luddendenfoot, Sowerby Bridge, Copley, Elland, Brighouse, Mirfield, Dewsbury and on to Wakefield.
The catchment lies on carboniferous rocks of Millstone Grit, and is heavily reservoired, with 39 reservoirs licensed to provide water. The river is joined by Hebden Water at Hebden Bridge, and is linked to the town of Rochdale, Greater Manchester across the Pennines via the Rochdale Canal.
Across much of its length, the Calder is canalised and becomes the Calder and Hebble Navigation. It is also part of the Aire and Calder Navigation, and to the east of Castleford, it merges into the River Aire, going on to join the River Humber and the North Sea.
The river was key to the success of the textile industries in the local area, and flows through the area known as the Yorkshire Heavy Woollen District. Many major mills were constructed along its banks, particularly at Halifax, Huddersfield, Dewsbury and Wakefield, but also in the smaller communities of Hebden Bridge, Sowerby Bridge and Todmorden. Many of these structures still exist as listed buildings, although the large scale production of yarn and textile has now ceased.
The river formed an important transportation system for raw materials and the products of the mills, particularly prior to the development of other infrastructures such as road and railway links to the area. At many places, the river is not navigable because of weirs or the shallow depth, and passage for boats was made by the creation of cuts where boats are able to enter the Calder and Hebble Navigation. Work began to make the Calder navigable above Wakefield in 1758.
At Wakefield, a variety of former mill buildings are currently being redeveloped to create a Waterfront project which will combine residential housing, offices, galleries and public spaces.
Until the 19th century, the Calder was home to large numbers of salmon but pollution from the textile and chemical industries along its banks led to the death of the salmon population by the mid 19th century. The last salmon on record was caught at Wakefield in 1850. Upstream of Huddersfield are popular areas for coarse fishing, roach, perch, chub, dace, minnows, gudgeon, pike, bream and trout. However, there are very little fish downstream of Huddersfield due to the amount of pollution produced by the town.
Alongside the river are four Sites of Special Scientific Interest.
Several Kingfishers are easily spotted (in Spring/Summer, due to its orange underbelly) at the section between Healey Mills and Horbury Bridge.
The river has been heavily polluted by the textile industry, and, more recently, chemical works along its banks.
Close to its source at Heald Foot, the water is polluted by the remains of past opencast mining activities and a landfill site. This has also led to a significant amount of soil erosion, with major mudslides into the river and its tributaries in 1947, 1982, 1991 and 2001.
As the river reached Huddersfield, the traditional textile industries created considerable amount of water pollution, particularly through the processes involved in the production of chemical dye and in scouring the wool clean. The enormous growth of the population over the last hundred years (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
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.
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. Lately there has been large amounts of rain which causes the risk of the River Calder flooding more common.
The river is home to a variety of watersports activities: