LANDSLIDES DRIVE DRAINAGE DIVIDES TOWARD STEADY STATE

Drainage divides are transient features that migrate in response to tectonic and erosional forcing, redistributing erosive energy by exchanging catchment area between basins. Although divide migration plays a central role in the long-term evolution of mountain belts, the controls on and rates of this process are poorly constrained. Earthquakes and intense storms in mountainous regions may generate thousands of landslides, some of which breach ridges and drive divide migration. The 2008 Wenchuan and 2015 Gorkha earthquakes in China and Nepal, and 2009 Typhoon Morakot in Taiwan, in total generated ~100,000 landslides between them. From these events, we mapped 365 landslides that caused divide migration resulting in a total exchange of ~2 km² between basins from ~82,000 km² affected by landsliding. Using this dataset of ridge-breaching landslides, we find evidence of coupling between river incision and drainage divide migration by applying several proposed proxies for divide stability based on river channel morphology. At the timescale of a single landslide-generating event, patterns of drainage area gain and loss among basins are consistent with divides generally migrating toward steady-state configurations, as inferred by channel metrics. We propose a new metric to quantify the mobility of divides and the contribution of a single landslide-generating event to a landscape’s progress toward steady state. Using this metric, we estimate that storms with a magnitude and recurrence similar to Typhoon Morakot generate landslides that are responsible for a minimum of 12-15% of southern Taiwan’s progress toward topographic steady state.