Hydraulic erosion
A terrain technique that simulates water flowing, tearing loose sediment, carrying it along and depositing it lower down — carving valleys and ridges into a heightmap.
Raw procedural noise gives hills and valleys but no trace of how real terrain is shaped: by water that pools, follows the slope and wears things down. Hydraulic erosion runs that process over a heightmap. Rain is spread across the surface, the water flows downhill, picks up material in proportion to its speed and slope and drops it again as the flow slows — valleys deepen, basins fill and lifelike river channels emerge.
The method dates to Musgrave, Kolb and Mace in 1989, in "The Synthesis and Rendering of Eroded Fractal Terrains". Two families exist: particle-based, which drops thousands of droplets and lets each gouge its own path, and grid-based, which solves a water layer cell by cell. Its sibling, thermal erosion, instead slumps material steeper than the angle of repose. Together they turn a diamond-square heightmap into something that looks weathered by rain.