Tributaries
The two most important tributaries of the Lahn, and those with the largest catchment inflows, are the Ohm and the Dill. The Dill originates in the southwestern foothills of the Rothaargebirge (the Haincher Höhe) and enters the Lahn from the right. The Ohm flows from the Vogelsberg and enters from the left. It is notable that not only is the Ohm at the point of its confluence with the Lahn only 1 km shorter from its source than the Lahn itself, but the Ohm’s catchment area (984 km2) is significantly larger than that of the Lahn above the confluence (652 km2, or only 452 km2 before the inflow of the Wetschaft only 2 km upstream).
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.
Because the highest point of the Westerwald is near the Sieg, and especially because the Taunus is very close to the Main, both Mittelgebirge are each considerably more than half drained by the Lahn. Especially the left tributaries from the Taunus flow with a strong south-north orientation. The river Emsbach runs through the Idstein Basin, which divides the (Hinter-) Taunus into two parts, while the Aar is central for the (Western and Eastern) Aartaunus.
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