Below Diez, the Lahn absorbs the Aar from the south.
At Fachingen in the municipality of Birlenbach, it leaves the Limburger Basin and enters the Lower Lahntal.
 Its course is incised over 200 metres (660 ft) deep in the Slate Mountains.
Near Obernhof, the Gelbach enters the Lahn opposite Arnstein Abbey.
Then, after passing Nassau and Bad Ems, where, as in Fachingen, mineral springs (sources of Emser salt) can be found, it completes its 242 km (150 mi) run, entering the Rhine in Lahnstein, located five kilometers south of Koblenz at an elevation of 61 metres (200 ft).
Below Diez, the Lahn absorbs the Aar from the south. At Fachingen in the municipality of Birlenbach, it leaves the Limburger Basin and enters the Lower Lahntal.  Its course is incised over 200 metres (660 ft) deep in the Slate Mountains. Near Obernhof, the Gelbach enters the Lahn opposite Arnstein Abbey. Then, after passing Nassau and Bad Ems, where, as in Fachingen, mineral springs (sources of Emser salt) can be found, it completes its 242 km (150 mi) run, entering the Rhine in Lahnstein, located five kilometers south of Koblenz at an elevation of 61 metres (200 ft).
Lahn