Knickpoint formation, rapid propagation, and landscape response following coastal cliff retreat at the last interglacial sea-level highstand: Kaua‘i, Hawai‘i

Upstream knickpoint propagation is an important mechanism for channel incision, and it communicates changes in climate, sea level, and tectonics throughout a landscape. Few studies have directly measured the long-term rate of knickpoint retreat, however, and the mechanisms for knickpoint initiation are debated. Here, we use cosmogenic ^3He exposure dating to document the retreat rate of a waterfall in Ka’ula’ula Valley, Kaua‘i, Hawai‘i, an often-used site for knickpoint-erosion modeling. Cosmogenic exposure ages of abandoned surfaces are oldest near the coast (120 ka) and systematically decrease with upstream distance toward the waterfall (