The Diemel is a river in Hesse and North Rhine-Westphalia, Germany. It is a left tributary of the Weser.RouteThe source of the Diemel is near Willingen, in Sauerland.The Diemel flows generally northeast through the towns Marsberg, Warburg, and Trendelburg. It flows into the Weser in Bad Karlshafen.The total length of the Diemel is 106 km.