Between the Lahn's source area in the Rothaargebirge and Gießen, all of the left tributaries are from the less mountainous parts of the West Hessian Bergland. After the turn towards the west or southwest near Gießen, all the left tributaries flow from the Hochtaunus. The right tributaries between the source area and the confluence of the Dill near Wetzlar come from the Gladenbach Bergland, while downstream they originate in the (High) Westerwald. Much of the Westerwald, in contrast, has no significant watershed, so the streams are almost random in finding their direction.