Partial landslide occlusion of a valley river and the hydro‐climatological drivers of landslide‐lake ephemerality

Abstract Understandings of landslide derived hazards have been of growing interest in response to increasing threats posed by such catastrophic mass movement events. Landslide lakes are of particular interest because of their ability to disrupt upstream and downstream river morphology posing potential catastrophic outburst flooding hazards. Landslide‐lake systems are often described as stable or persistent features, or unstable dams with catastrophic failure. Here, we document an intermediate condition in which a landslide‐lake switches between presence and absence over sub‐decadal timescales using the case study of Te Horo in the Southern Alps/Ka Tiritiri o te Moana of New Zealand. A chronology of lake presence was constructed from remotely sensed imagery identifying 42 hydrological lake filling and draining instances over a decade. Impoundment following river channel occlusion was initiated in late 2013, and the subsequent 11‐year chronology demonstrates a lake transience or pulsing condition. Following formation of Te Horo in 2013, the landslide lake grew to a maximum of 1.4 km 2 and persisted until early 2019, generally decreasing in size over this period. Pulsing lake behaviour (months to days) began between 2019 and 2020, punctuated by the largest streamflow event on record (2250 m 3 s −1 ) in February 2020 generating the longest impoundment duration in the pulsing stage (427 days). We classified lake filling events into three types: high magnitude rainfall and high‐flow events (Type I), lake filling driven by antecedent rainfall conditions without a significant high‐flow event (Type II), and events with no significant hydrological inflows, attributed to changes to landslide toe morphology (Type III). Across 42 discrete lake filling events between 2019 and 2025, Type I accounted for 32%, Type II for 37% and Type III for 31%. We interpret the prevailing pulsing of lake presence and absence to be an intermediate state of landslide dam occlusion when such streamflow‐driven filling events occur in pluvially dominated and wide valley morphology (>0.5 m), where high river competence is effective at maintaining the drainage channel.