HOWTO generate erosion to your heightmap.
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
Filter → Fill → Fill Basins (CTRL-B), use default -1 value
Filter → Mathematical → Span, Low = -60, High = 1000
Filter → Noise → Percentage Noise, 5% (5% is very very rough, too rough! suggest you try smaller values or add some blur later)
Filter → Fill → Fill Basins (CTRL-B), use default -1 value
Choose incise flow or fluvial erosion first pass, see below
Filter → Fill → Fill Basins (CTRL-B), use default -1 value
Choose incise flow or fluvial erosion second pass, see below
That makes quite nice looking eroded terrain.