Waterfall height sets the mechanism and rate of upstream retreat

Waterfalls are among the fastest-eroding parts of river networks, but predicting natural waterfall retreat rates is difficult due to multiple processes that can drive waterfall erosion. We lack data on how waterfall height influences the mechanism and rate of upstream waterfall retreat. We addressed this knowledge gap with experiments testing the influence of drop height on waterfall retreat. Our experiments showed that shorter waterfalls retreat up to five times faster than taller waterfalls, when bedrock strength, sediment supply, and water discharge are constant. This retreat rate difference is due to a change in the erosion mechanism. Short waterfalls retreat by the formation of several small, rapidly eroding bed-rock steps (i.e., cyclic steps), whereas tall waterfalls tend to form large bedrock plunge pools where lateral plunge pool erosion allows headwall undercutting and subsequent waterfall retreat. Because waterfall height can be partially set by the waterfall formation mechanism, our results highlight that the rate of waterfall retreat and subsequent landscape evolution can be modulated by the processes that form waterfalls.