Controls of climate and fluvial thresholds on stream incision on the Hawaiian Islands

The extent that climate impacts erosion has long been debated and is difficult to decipher, as climate signals are often obscured by tectonic signals. The Hawaiian Islands are excellent settings to test these relationships, as they have no tectonic uplift, well-known base level histories, uniform lithology, and dramatic climate gradients. On Kauai, previous work shows a correlation between rainfall and total canyon incision; however, geologic evidence shows that river incision could not have been constant throughout time: ~2 Ma inset lava flows sit at river level in several canyons on the ~4.5-million-year-old island, indicating that canyon incision has been negligible over the last 2 Myr. One explanation is that boulders and/or sediment flux in streambeds block further incision, acting as thresholds. Channel gradients on the Hawaiian Islands decrease with increasing precipitation and appear to be in equilibrium (uniform k_sn below knickpoints), indicating that channel gradient is set by the threshold of boulder or sediment mobility. Therefore, climate appears to be recorded in threshold channel slope and total incision depth instead of time averaged incision rate as previously assumed (Ferrier et al., 2013). We traveled to Kaui and West Maui to test the hypotheses that (a) boulders and/or (b) sediment act as thresholds for fluvial incision, by (a) quantifying channel morphology and boulder size using structure from motion photogrammetry, drone photography and pebble counts, and (b) collecting catchment-average samples for cosmogenic radionuclide analysis (³⁶Cl in magnetite), in catchments across gradients of precipitation rate and channel slope. Preliminary data suggests that boulder size scales with channel gradient within climate zones, indicating that boulders are an important threshold for channel incision. However, if channel gradient is correlated with upcoming denudation rates, sediment flux may be an additional important threshold. Next steps include quantifying basal and critical shear stress for boulder transport and combining data into a landscape evolution model. Ultimately, we aim to answer long-standing questions regarding the roles of climate and fluvial thresholds in landscape evolution, applicable in both tectonically inactive and active landscapes.