Knickpoint formation, rapid propagation, and landscape response following coastal cliff retreat at the last interglacial sea-level highstand: Kaua'i, Hawai'i

Abstract

Upstream knickpoint propagation is an important mechanism for channel incision, and it communicates changes in climate, sea level, and tectonics throughout a landscape. Few studies have directly measured the longterm rate of knickpoint retreat, however, and the mechanisms for knickpoint initiation are debated. Here, we use cosmogenic 3 He exposure dating to document the retreat rate of a waterfall in Ka'ula'ula Valley, Kaua'i, Hawai'i, an often-used site for knickpointerosion modeling. Cosmogenic exposure ages of abandoned surfaces are oldest near the coast (120 ka) and systematically decrease with upstream distance toward the waterfall (<10 ka), suggesting that the waterfall migrated nearly 4 km over the past 120 k.y. at an average rate of 33 mm/yr. Upstream of the knickpoint, cosmo genic nuclide concentrations in the channel are approximately uniform and indicate steady-state vertical erosion at a rate of ~0.03 mm/yr. Field observations and topographic analysis suggest that waterfall retreat is dominated by block toppling, with sediment transport below the waterfall actively occurring by debris fl ows. Knickpoint initiation was previously attributed to a submarine landslide ca. 4 Ma; however, our dating results, bathymetric analysis, and landscape-evolution modeling support knickpoint generation by wave-induced seacliff erosion during the last interglacial sealevel highstand ca. 120-130 ka. We illustrate that knickpoint generation during sea-level highstands, as opposed to the typical case of sea-level fall, is an important relief-generating mechanism on stable or subsiding steep coasts, and likely drives transient pulses of signifi cant sediment fl ux.

New insights into the mechanics of fluvial bedrock erosion through flume experiments and theory

“…Erosion at waterfalls is important to understand because waterfalls can be the fastest eroding parts of some landscapes, and they can communicate changes in tectonics, climate, and sea-level throughout a drainage basin (e.g., Gilbert, 1907;Howard et al, 1994;DiBiase et al, 2014;Mackey et al, 2014). Some workers have applied saltation-abrasion theories to steep river reaches and waterfalls (e.g., Chatanantavet and Parker, 2006;Wobus et al, 2006;Crosby et al, 2007;Gasparini et al, 2007;Goode and Burbank, 2009); however, these theories are unlikely to hold for high gradient rivers.…”

“…14). This may explain the prevalence of stepped (rather than holey) topography in pluckingdominated landscapes and signifies the important role of knickpoint retreat in landscapes with well-jointed rock (e.g., Miller, 1991;Weissel and Seidl, 1997;Mackey et al, 2014). Thresholds for block sliding are similar to sediment transport, such that the limiting factors on erosion rates where sliding is dominant may vary from place to place depending largely on the block friction angle ( ).…”

Bedload transport controls bedrock erosion under sediment-starved conditions

“…For comparable situations where mean values of bedload volumes and erosion rate on bare bedrock sections are available, this model can be easily calibrated by adjusting its prefactor K. Potential applications are steep bedrock channels in detachment-starved catchments (e.g., Wohl, 1998Wohl, , 1999, channel knickpoint sections with exposed bedrock such as waterfalls (e.g., Miller, 1991;Wohl et al, 1994;Cook et al, 2013;Mackey et al, 2014;DiBiase et al, 2015), high lateral bedrock sections above the channel or its banks (e.g., Hartshorn et al, 2002;Turowski et al, 2008) or even hydropower facilities that have to cope with natural sediment flux such as sediment bypass tunnels (Jacobs and Hagmann, 2015).…”

Erosion during extreme flood events dominates Holocene canyon evolution in northeast Iceland

“…If a knickpoint is retreating steadily through time, as would be expected if associated with a normal flow regime, the exposure age of samples from fluvial surfaces would become progressively younger with decreasing distance from the modern knickpoint location (28)(29)(30). If, on the other hand, a knickpoint retreated a large distance in a short period, such as during an extreme flood event, the exposure ages would be expected to cluster around the time of that significant erosion event (19).…”