PMC Editing Wiki: Tools, Wilbur

HOWTO generate erosion to your heightmap.

1. Filter → Noise → Fractal Noise, ridged multifractal, operation: replace, H: 1, amplitude: 1, lacunarity: 1.9, displacement: -10, octaves: 7, XY scale: 2 and other box 2, offset: 1, XY origin: -10 and other box -10, fgain: 2, seed: click random button
2. Filter → Fill → Fill Basins (CTRL-B), use default -1 value
3. Filter → Mathematical → Span, Low = -60, High = 1000
4. Filter → Noise → Percentage Noise, 5% (5% is very very rough, too rough! suggest you try smaller values or add some blur later)
5. Filter → Fill → Fill Basins (CTRL-B), use default -1 value
6. Choose incise flow or fluvial erosion first pass, see below
7. Filter → Fill → Fill Basins (CTRL-B), use default -1 value
8. Choose incise flow or fluvial erosion second pass, see below

That makes quite nice looking eroded terrain.