Numerous smaller and larger weirs still hinder the migration of fish and other aquatic organisms in the Aar, the Palmbach or in the Mühlbach and Gelbach. Many fish species, however, depend during their life cycle, e.g. for the spawning season, from reaching specific habitats in the river.

Weirs prevent fish from migrating into the upstream river sections. In order to allow for a free movement of the fishes, both up and downstream, it is foreseen to revise several dams at tributaries to the Lahn in an ecologically friendly way.