Abstract: An important, largely unexplored area of computer image generation is the simulation of weathering and its effects on appearance. Weathering results from the interaction of the environment with the materials in the world. The flow of water is one of the most pervasive and important natural forces involved in the weathering of materials, producing a distinctive set of patterns of washes and stains. This paper presents an intuitive phenomenological model for the flow of water over surfaces that is capable of generating such changes in appearance. We model the flow as a particle system, each particle representing a "drop" of water. The motion of the water particles is controlled by parameters such as gravity, friction, wind, roughness, and constraints that force the particles to maintain contact with the surface. The chemical interaction of the water with the surface materials is governed by a set of coupled differential equations describing both the rate of absorption of water by the surface and the rate of solubility and sedimentation of deposits on the surface. To illustrate the power of this simple model, we show examples of flows over complex geometries made from different materials; the resulting patterns are striking and very difficult to achieve using traditional texturing techniques.