Upstream migration of avulsion sites on lowland deltas with river-mouth retreat

River deltas are fertile, populous landscapes that grow through river avulsions-episodic channel-jumping events. Historically, avulsions on deltas occurred at persistent locations, causing some of the deadliest recorded floods. Climate change and human activities are threatening to drown deltas but it is unknown how avulsions will respond because they occur infrequently on large, lowland deltas. Here, we use a low-gradient river delta formed on the margin of a lake in the Qaidam Basin, China, as a natural laboratory to explore how lowland deltas will respond to river-mouth retreat from accelerated relative sea-level rise. Using satellite imagery from 1973 to 2010 C.E., we identified and analyzed the response of 6 lobe-scale avulsions on the Sulengguole River delta to the seasonal expansion of the North Huoluxun Lake in the Qaidam Basin. We show that the seasonal lake-water area increase caused punctuated river-mouth retreat. In response, avulsion sites migrated upstream at a commensurate rate such that the avulsion length-streamwise distance of avulsion site to the river mouth-remained consistent and scaled with the backwater lengthscale, similar to large, lowland deltas. Results indicate that the drowning of lowland deltas from accelerated relative sea-level rise will shift avulsion hazards tens-to-hundreds of kilometers upstream, exposing new upstream communities to the risk of catastrophic flooding. (C) 2021 Published by Elsevier B.V.