Models for river avulsions have identified the ratio between down-valley and cross-valley slopes of channels as the triggering factors for the sudden channel shift but have remained untested in the field. The August 2008 avulsion of the Kosi River at Kusaha, 12 km upstream of the Kosi barrage in Nepal, provided an opportunity to study a large-scale avulsion (~120 km) for its causal factors and driving mechanisms.