The limestone massif of the Brilon plateau is crisscrossed by many fissures. The precipitation seeps into them and then runs off underground. Thus, only a few slightly depressed valley forms have developed, which are almost permanently dry. There are no permanently flowing waters. This is because as soon as the streams from the surrounding mountains reach the Brilon plateau, they suddenly disappear.
The swales or Bachschwinden, some of which have sunk 10 m into the plateau, can be precisely located almost everywhere. The sinking brooks have formed valley closures at the swallow holes, which not infrequently have the form of upstream open amphitheaters. During heavy precipitation or snowmelt, they are sometimes unable to contain the water and impoundments occur. Small lakes then form for a short time.