For a reservoir-type simulation, a pre-processing step is to determine whether water in the reservoir would progress downstream if the impounding structure(s) were to be removed entirely. This is accomplished by flood-filling an area with the cells in the footprint of the impounding structures virtually brought to their respective crest elevations, and then flood-filling again after bringing them down to their respective toe elevations. Depending on the difference in number of cells filled between the two, it may be determined that the impounding structures do not adequately allow the water from the reservoir to flow downstream. This may be due to the orientation of the given impounding structure crest lines, the footprint of the structures (determined by the hydraulic height and structure type), or elevations downstream which prevent the water from propagating.