Simulated raindrops flow downhill, picking up sediment and depositing it elsewhere. Repeated millions of times, this carves realistic valleys, ridges, and river channels from random noise terrain.
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Algorithm (Hans Theobald Beyer 2006): Each drop has position, velocity, and sediment load. It accelerates downhill (gradient), picks up sediment proportional to speed×capacity deficit, deposits when speed decreases. Evaporation stops the drop. Thousands of drops sculpt realistic terrain.