Limits to knickzone retreat and bedrock river incision on the Hawaiian islands.

A knickzone is defined as a waterfall or an oversteepened fluvial reach encompassing a series of waterfalls. Knickzones are remarkable landforms in that they may represent diametrically opposed conditions: either rapid upstream propagation of base-level fall or a condition of stability and stalled response to base-level fall. Knickzones on the Hawaiian islands exhibit evidence of both behaviours, and in this study, we explore whether this dichotomy can be explained by a threshold stream power for river incision. Topographic analysis shows that the transition between fluvial hanging valleys, where no measurable upstream retreat from the stream junction or coastline has occurred, and knickzones that have retreated or formed upstream of their outlet can be defined by a range of catchment sizes from ~0.5 to 6 km². This transition is present on all volcanoes in our analysis regardless of significant base-level fall, and there is no clear trend of drainage area above knickzones with volcano age. To explain these observations, we hypothesize that knickzones form or stabilize where maximum unit stream power (ω_m) does not exceed the critical unit stream power required to incise bedrock (ω_c). Using estimates of a maximum possible flood discharge as a function of drainage area to calculate ω_m, we show that drainage areas above knickzones are well described by the theoretical prediction that incision stalls when ω_m ≤ ω_c, where ωc falls in a narrow range from 14 to 40 kW/m². Furthermore, we demonstrate that knickzone positions are largely insensitive to mean climate conditions, likely reflecting the fact that extreme flood events are either insensitive or inversely correlated with mean annual rainfall. [ABSTRACT FROM AUTHOR]