The River Calder is a river in Cumbria, England.
The river rises at Lankrigg Moss and flows southwards through an ancient landscape, flowing under Monks Bridge and by the site of Calder Abbey, as well as several tumuli and other mysterious monuments.
However, in modern times, the river runs straight through the site of Sellafield nuclear power plant where it gives its name to Calder Hall, site of the world's first commercial nuclear reactor. It flows into the Irish Sea at the same point as the River Ehen, just southwest of Sellafield.