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

Abstract

AbstractA 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 ( ) does not exceed the critical unit stream power required to incise bedrock . Using estimates of a maximum possible flood discharge as a function of drainage area to calculate , we show that drainage areas above knickzones are well described by the theoretical prediction that incision stalls when , where 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.