Sculpt a landscape and watch it move. Set rainfall, soil erodibility, vegetation cover and steepness — the runoff erodes the hills, transport by flowing water sorts/deposits the sediment downstream, and gullies carve themselves. The universal soil loss equation (USLE) predicts the rate; the terrain evolution emerges from the physics.
Soil loss is predicted with the Universal Soil Loss Equation A = R·K·L·S·C·P where R = rainfall erosivity, K = soil erodibility (0.05–0.55), LS = slope-length factor (LS ≈ √(L/22)·(0.065+0.045·S+0.0065·S²), the classic Wischmeier–Smith formula), C = cover/management and P = conservation practice. In the terrain evolution, each cell erodes at ~A and the sediment is routed downhill by water: deposition happens where slope flattens — exactly why valleys silt up and floodplains form. Vegetation cover cuts erosion roughly like exp(−0.035·C%) and, in the sim, also knits the surface together so gullies struggle to start. Gullying risk fires when local slope × runoff exceeds a threshold — that's where the landscape develops its most dramatic scars.