Résumé
In the Himalayas of central Nepal, it is now widely accepted that the consistency between present-day vertical velocities and both denudation and fluvial incision rates across the range implies that erosion should be in equilibrium with tectonic uplift. However recent studies in both Sikkim and Bhutan reveal a more complex behavior. In this part of the Himalayas, denudation does not compare well with river incision rates along the main rivers, suggesting either a temporal evolution of erosion processes over the last approximately 50 kyr or a non-steady state topography. Here we use a 90-m-resolution SRTM digital elevation model to test whether the transition between these two parts of the Himalayas coincides with lateral variations of geomorphological parameters. Several parameters such as roughness, mean relief, local slope or mean basin elevation exhibit no clear lateral variation. Following Duncan et al. (2003), our results confirm that the Himalayas of central Nepal have specific features with a low hypsometric integral and a low mean elevation associated with high river entrenchment. Furthermore, our results suggest a scaling dependence of the drainage density with respect to a critical threshold drainage area. Below this threshold, the drainage density appears as a highly variable parameter, whereas above it, drainage density and drainage area appear to be independent. We interpret this threshold as a transition from small drainages with low stream power, to large drainage areas where stream power is high enough to produce erosion rates significant enough to equilibrate with rock uplift. Even though we cannot rule out that parts of the Himalayas are not in topographic steady-state, this new criterion suggests that any disagreement between denudation and incision rates may be related to the size of the documented watersheds.