When knickzones limit upstream transmission of base-level fall: An example from Kaua‘i, Hawai‘i

A threshold drainage area limits fluvial transmission of base-level fall and may be expressed in the form of a waterfall or a series of waterfalls, defined here as a knickzone. Knickzones on the west coast of Kaua‘i, Hawai‘i (USA), exhibit evidence of a threshold drainage area. Eighteen (18) of the 25 knickzones in our study area are located at the coast or a tributary junction, have a drainage area <1.5 km2, and have been stationary for at least 1.5 m.y. The other seven knickzones are located >1 km upstream from the coast or nearest tributary junction and range in drainage area from 1 to 5.5 km2. Both sets of knickzones limit incision relative to canyons without knickzones. Field observations show strong ‘a‘ā flows and dikes always crop out at the lip of knickzones, suggesting these resistant rocks and coarse sediment generated from them act to inhibit knickzone migration. A model incorporating flood records and channel conditions above knickzones shows thresholds of coarse sediment entrainment are never exceeded below 1 km2. Our results demonstrate knickzones on the west coast of Kaua‘i are enduring features explained by resistant lava flows and physical limits in bedrock incision.