The River Calder is a river in West Yorkshire, in Northern England. The Calder rises on Heald Moor in Lancashire close to the source of another river with the same name, and then flows east into West Yorkshire through green countryside, former woollen-mill villages and towns before joining the River Aire near Castleford. The river's valley is generally known as the Calder Valley. The name Calderdale usually refers 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, Hebble Brook and the 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" (e.g. 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. Etymology The name 'Calder' is thought to come from the early Brythonic, meaning 'hard or violent water' (compare Modern Welsh caled "hard"), or possibly from another Celtic language, meaning river of stones. This history is reflected in the name of a village, Walsden, in upper Calderdale, which is probably derived from Wales Dene, or "Valley of the "Welsh" ('Welsh' here referring to all 'Brythonic Celts') in Anglo-Saxon. Geography The river rises approximately above sea level at Heald Moor, north-west of Todmorden, and drains an area of . It flows for a distance of around through Cornholme, Todmorden, Eastwood, Hebden Bridge, Mytholmroyd, Luddendenfoot, Sowerby Bridge, Copley, Elland, Brighouse, Mirfield, Dewsbury, Horbury Bridge 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 by the River Ryburn at Sowerby Bridge; it is linked to the town of Rochdale, Greater Manchester across the Pennines via the Rochdale Canal. For 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