KNICKPOINTS OF KAUA'I, HAWAI’I: ACCELERATED INCISION OR LITHOLOGICAL CONTROL?

Vertical-step knickpoints (waterfalls) in bedrock channels are often argued as evidence of acceleration in incision rate following an abrupt change in base level. However, observations show that resistant lithologies are often exposed at the lips of waterfalls or vertical-step knickpoints. Over the past decade understanding the mechanisms that drive knickpoint retreat has been a major focus of the community, with significant progress in developing mechanistic models of this important process. However, the question of whether the presence of a vertical-step knickpoint actually indicate an accelerated incision rate remains open to debate. Here we contribute to this discussion by examining the long-standing hypothesis that a vertical-step knickpoint can simply be an expression of a more resistant band of bedrock that actually causes a long-term reduction in incision rate. For testing and analysis of this hypothesis we turn to Kaua’i, Hawai’i where adjacent canyons have experienced the same base level conditions, the same climate, and are characterized by the presence or absences of major vertical-step knickpoints and are composed of spatially variable bands of a’a and pahoehoe flows. Through topographic analysis, field observations, and modeling we find evidence that while base level change induces a period of accelerated incision and knickpoint migration, the preservation of vertical-step knickpoints often record a slowing or stalling of this process due to more resistant lithologies. Subsequently, resistant layers are frequently associated with knickpoints and importantly limit the total amount of incision, the long-term incision rate, and act as a mechanism for preservation of initial surfaces.