The emergence of an orogen is generally characterized by an early phase of rapid surface uplift and the concomitant evolution of montane topography, leading in some cases to a steady state in which tectonic mass flux is balanced by erosion. This early phase of mountain growth is exemplified in the Finisterre Mountains of Papua New Guinea, a propagating growth fold whose catchments can be observed at a range of stages in their temporal evolution. Watersheds appear to initiate by isolated gorge incision, to expand by large-scale landsliding in a manner controlled by ground-water seepage, and to entrench by fluvial incision of landslide scars and deposits. Once a montane system of ridges and valleys is established, only rare, major landslides can modify the drainage pattern. The steady-state morphology of a mountain belt is therefore intimately related to its initial phase of growth.