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) erosion_0_tn.jpg erosion_0_3d.jpg
30% erosion_30_tn.jpg erosion_30_3d.jpg
50% (centre, default) erosion_50_tn.jpg erosion_50_3d.jpg
70% erosion_70_tn.jpg erosion_70_3d.jpg
100% (right, strong) erosion_100_tn.jpg erosion_100_3d.jpg