Timescales of landscape response to divide migration and drainage capture: Implications for the role of divide mobility in landscape evolution

Efforts to extract information about climate and tectonics from topography commonly assume that river networks are static. Drainage divides can migrate through time, however, and recent work has shown that divide mobility can potentially induce changes in river profiles comparable to changes caused by variation in rock uplift, climate, or rock properties. We use 1-D river profile and 2-D landscape evolution simulations to evaluate how mobile divides influence the interpretation of river profiles in tectonically active settings. We define a nondimensional divide migration number, N-Dm, as the ratio of the timescale of channel profile response to a change in drainage area (T-dA) to the timescale of divide migration (T-Dm). In simulations of headward divide migration, N-Dm is much less than unity with no measurable perturbation of channel profiles. Only in simulations configured to induce rapid lateral divide migration are there occasional large stream capture events and zones where localized drainage area loss is fast enough to support N-Dm values near unity. The rapid response of channel profiles to changes in drainage area ensures that under most conditions profiles maintain quasi-equilibrium forms and thus generally reflect spatiotemporal variation in rock uplift, climate, or rock properties even during active divide migration. This implies that channel profile form may not reliably record divide mobility, so we evaluate alternate metrics of divide mobility. In our simulations and an example in Taiwan, we find that simple measures of cross-divide contrasts in topography are more robust metrics of divide mobility than measures of drainage network topology.