The influence of surface slope on the shape of river basins: Comparison between nature and numerical landscape simulations

We investigate the influence of initial conditions of slope and surface roughness on the shape (length to width aspect ratio) of incipient drainage basins in numerical experiments of simple tilted surfaces using the CASCADE code of landscape evolution. Comparison with data on the shape of river basins in nature shows that simple rules of the steepest-descent routing of water are sufficient to account for a natural range of incipient drainage basin shape, independently of the erosion processes at work. To produce numerical basins that respect the main aspect ratio of natural drainage basins, one must use very low initial regional surface slopes of less than 1° at the scale of the entire drainage basins, and a local roughness slope of less than 3° at the scale of local surface irregularities. Numerical studies addressing real study cases may take advantage of the relation between local roughness and regional slope in order to produce catchment aspect ratios similar to the natural studied cases.