Blocked by boulders: Examining the roles of climate, fluvial thresholds and sediment flux in slowing bedrock incision on the Hawaiian Islands

Tectonic uplift is the primary control on bedrock river incision; however, in places where uplift is absent, such as the Hawaiian Islands, other factors including climate, lithology, sediment flux, fluvial thresholds, baselevel history, and time become crucial to understanding the form and incision history of streams. Ferrier et al. (2013) argued that streams on Kauai show a linear relationship between unit stream power and incision rate with a negligible incision threshold. However, large boulders that choke Kauai’s streams suggest significant incision thresholds, and young lavas at river level suggest that incision has been negligible for ~2 Myr. We suggest that these channels are at a critical threshold set by climate, fluvial thresholds, and sediment flux that prevents them from further incising into bedrock. We present 7 new Ar-Ar ages of young lavas emplaced within Kauai’s canyons along with an extensive grain size dataset spanning 22 catchments, and 17 new catchment-averaged erosion rates (36Cl in magnetite) from catchments on Kauai and West Maui. Our results show that the young lavas were emplaced at 1-2 Ma, confirming negligible modern bedrock incision. The size of boulders in streams (D84 from gridded point counts and “area-by-area” analysis) covaries with normalized local channel gradient (ksnQ), suggesting that sediment size determines the threshold for motion and that these boulders can block bedrock incision. Finally, catchment-average erosion rates are weakly correlated with MAP, ksnQ and valley wall gradient, suggesting that both fluvial thresholds and sediment flux, modulated by local climate, play a role in setting channel steepness and slowing bedrock incision on the Hawaiian Islands.