Its catchment area is mainly on its left side, because in its southern upper valley it has a strong competitor on the right side in the nearby Wieslauf and its tributaries with a clearly deeper erosion base, in the eastern middle course to Heuchlingen the Rems tributaries dig the water out of it so much that the watershed follows the Lein itself closely in the south, often only a few hundred meters away from the river itself.

Only in the northeast-oriented lower course from Heuchlingen it has any noteworthy right tributaries at all, here it shares the ridge around Dewangen with the Kocher in about equal parts.

From the source to the estuary: Hydrologically, the Lein can be considered the main source of the Kocher system, as it is more than twice as long at the mouth of the river as this one (57 km vs. 25 km), whose name overflow also exceeds in the catchment area (250 km2 vs. 152 km2) and also carries slightly more water than this one - despite its strong karst springs, which additionally feed from areas beyond its superficial watersheds.

The reason for this is that the course of the river Lein was laid out at a time when the area was still draining to the southeast towards the Danube and the Black Sea.

Through numerous stream captures of Danube tributaries the European main watershed shifted gradually to the southeast in favour of the Rhine.