Erosion (DM parameter)

The 'erosion' slider in the design map parameters wizard determines the amount of fluvial and thermal erosion that will be applied to your heightmap.

This is demonstrated below, where where I've made terrain with erosion settings from 0% up to 100%:

Erosion	Heightfield	Terrain normals	3D render
0% (left, none)
30%
50% (centre, default)
70%
100% (right, strong)