Abstract: In this paper we present interactive techniques for physics-based simulation and realistic rendering of rivers using Smoothed Particle Hydrodynamics. We describe the design and implementation of a grid-less data structure to efficiently determine particles in close proximity and to resolve particle collisions. Based on this data structure, an efficient method to extract and display the fluid free surface from elongated particle structures as they are generated in particle based fluid simulation is presented. The proposed method is far faster than the Marching Cubes approach, and it constructs an explicit surface representation that is well suited for rendering. The surface extraction can be implemented on the GPU and only takes a fraction of the simulation time step. It is thus amenable to real-time scenarios like computer games and virtual reality environments.