The now southward-flowing Lahn then enters the Marburg-Gießen Lahntal.
Shortly before Cölbe, the Ohm enters from the left at the Lahn-Knie named area.
Flowing from the Vogelsberg through the Ohmtal, the Ohm is the Lahn's longest tributary, with a length of 59.7 kilometres (37.1 mi).

The river then breaks through a sandstone mesa (the Marburger Rücken to the west and the Lahnberge to the east) into a valley which encompasses the entire territory of the city of Marburg and its suburbs.
The valley begins after the river passes the Marburger Rücken near Niederweimar, where the Allna enters from the right.
At the valley's southern end, the Zwesten Ohm enters from the Lahnberge.
The right (western) side of the valley is again formed by the Gladenbacher Bergland, from which the Salzböde enters the Lahn.
On the left rises the Lumda Plateau, from which the eponymous river Lumda flows into the Lahn near Lollar.
Gradually the valley widens into the Gießen Basin.

In Gießen, after the inflow of the Wieseck from the left, the Lahn's general direction of flow changes from the south to the west.
The Gießen Basin extends a few more miles downstream to Atzbach, a suburb of Lahnau.
From the 1960s until the 1980s, there was extensive gravel mining in this area.
The area between Heuchelheim, Lahnau, and the Wetzlar borough of Dutenhofen was to be completely mined and a water sports center with an Olympic-suitable rowing course built.
This plan was partly realized, and the Heuchelheim Lake and Dutenhofen Lake are now popular recreational destinations for the surrounding region.
Nature conservation organizations, however, were able to prevent further gravel mining, so the area is now one of the largest nature reserves in Hesse.
Dutenhofen Lake marks Kilometer 0 of the Lahn as a federal waterway.

The Gießen Basin is surrounded by the mountain peaks of the Gleiberg, the Vetzberg, the Dünsberg, and the Schiffenberg.
 At Wetzlar, the Lahn is joined by its second longest tributary, the Dill, which has a length of 55.0 kilometres (34.2 mi).
At this location, the valleys of the Lahn and Dill separate three parts of the Rhenish Slate Mountains from each other: the Gladenbach Bergland, the Westerwald to the northwest, and the Taunus to the south.
The now southward-flowing Lahn then enters the Marburg-Gießen Lahntal. Shortly before Cölbe, the Ohm enters from the left at the Lahn-Knie named area. Flowing from the Vogelsberg through the Ohmtal, the Ohm is the Lahn's longest tributary, with a length of 59.7 kilometres (37.1 mi).
The river then breaks through a sandstone mesa (the Marburger Rücken to the west and the Lahnberge to the east) into a valley which encompasses the entire territory of the city of Marburg and its suburbs. The valley begins after the river passes the Marburger Rücken near Niederweimar, where the Allna enters from the right. At the valley's southern end, the Zwesten Ohm enters from the Lahnberge. The right (western) side of the valley is again formed by the Gladenbacher Bergland, from which the Salzböde enters the Lahn. On the left rises the Lumda Plateau, from which the eponymous river Lumda flows into the Lahn near Lollar. Gradually the valley widens into the Gießen Basin.
In Gießen, after the inflow of the Wieseck from the left, the Lahn's general direction of flow changes from the south to the west. The Gießen Basin extends a few more miles downstream to Atzbach, a suburb of Lahnau. From the 1960s until the 1980s, there was extensive gravel mining in this area. The area between Heuchelheim, Lahnau, and the Wetzlar borough of Dutenhofen was to be completely mined and a water sports center with an Olympic-suitable rowing course built. This plan was partly realized, and the Heuchelheim Lake and Dutenhofen Lake are now popular recreational destinations for the surrounding region. Nature conservation organizations, however, were able to prevent further gravel mining, so the area is now one of the largest nature reserves in Hesse. Dutenhofen Lake marks Kilometer 0 of the Lahn as a federal waterway.
The Gießen Basin is surrounded by the mountain peaks of the Gleiberg, the Vetzberg, the Dünsberg, and the Schiffenberg.  At Wetzlar, the Lahn is joined by its second longest tributary, the Dill, which has a length of 55.0 kilometres (34.2 mi). At this location, the valleys of the Lahn and Dill separate three parts of the Rhenish Slate Mountains from each other: the Gladenbach Bergland, the Westerwald to the northwest, and the Taunus to the south.
Lahn