Landslide-driven drainage divide migration

Drainage divide migration reorganizes river basins, redistributing erosive energy and contributing to feedbacks between tectonics, erosion, and climate. However, the conditions governing divide migration and the time scales on which it occurs are poorly understood. By connecting channels to hillslopes in steep landscapes, landslides are expected to play a central role in divide migration and landscape evolution. In this study, we examine landslides triggered by three events (two earthquakes and a tropical cyclone), seeking insight into controls on divide migration. Of the similar to 100,000 landslides triggered, we mapped 365 that caused a divide to migrate, resulting in a total exchange of similar to 2 km(2) between basins from similar to 82,000 km(2) affected by landsliding. By applying several proposed metrics for divide stability based on river channel morphology, we use our database of divide migrations to test for the role of landslides in coupling between channels and divides. We find that, at the time scale of a single landslide-generating event, patterns of area gain and loss between basins are consistent with landscapes progressing toward steady state, as inferred from channel metrics. We also propose a metric to quantify divide migration, area exchange, and the contribution of an event toward topographic steady state. Restricting our analysis to the main drainage divide, and using estimates of recurrence interval and the rate of topographic evolution in Taiwan, we calculate that landslides triggered by large typhoons account for a minimum of 12%-15% of southern Taiwan's progress toward steady state.