Under what conditions do parallel river networks occur?

Geologists have long recognized that channel networks can deviate from a typical dendritic form when they develop under certain geologic or topographic constraints. One such deviation is the parallel form, which is thought to occur when networks develop on sloping surfaces. The objectives of this research are to determine the specific conditions under which parallel networks occur and the nature of the transition between dendritic and parallel networks. Natural and simulated channel networks are considered in this study. The natural networks were obtained from the digital elevation models of basins that include remnants of the preexisting topographic surface. These remnants were identified as locations with small drainage areas and topographic curvatures that are close to zero. For each basin, the average slope of the preexisting surface was calculated, and the channel network was classified using an existing method that can distinguish five network types (including dendritic and parallel) based on measures that are derived from scaling invariance. The natural networks become consistently parallel when the average slope of the preexisting surface exceeds about 3%. Simulated channel networks were also generated using a detachment-limited model for fluvial erosion. The parameters of the model were determined to imitate the natural basins, and the average slopes of the observed preexisting surfaces were used for the slopes of the initial surfaces in the simulations. The model can also produce a transition between dendritic and parallel networks for an initial slope around 3%, but this transition depends on the roughness of the initial surface and the boundary conditions.