CANYON FORMATION ON THE HAWAIIAN ISLANDS: CAN A SINGLE THRESHOLD OF RIVER INCISION EXPLAIN OBSERVED PATTERNS OF INCISION?

Complex interactions between climate, tectonics, and lithology drive the evolution of the Earth’s surface, with rivers setting the pace and pattern of landscape response to changes in base-level fall and climate. Efforts to capture this key role of rivers in landscape evolution have led to numerous models that range in complexity and proposed mechanisms of incision. The stream power incision model (SPM) accounts for 1^st order controls on bedrock incision, striking a balance between ease of calibration and incorporating the role of floods only through the concept of a dominant, “geomorphically effective” flood. However, observed non-linear scaling of erosion rates with channel steepness may reflect the interactions of incision and transport thresholds with the stochastic distribution of floods not captured in the SPM. In addition, recent work has identified potential negative feedbacks on incision where large hillslope-derived boulders increase these thresholds. Evaluating the degree to which these controls are important remains challenging, especially since most studies invoking the SPM focus on tectonically active regions with difficult-to-assess and difficult-to-isolate factors. To better identify the role of thresholds in river incision we turn to the islands of Moloka’i and Maui, Hawai’i, where initial volcanic surface slope and rainfall vary systematically. Canyons range in incision depth from 10s of meters up to 1km. Preliminary analysis suggests that these patterns of incision depth can adequately be captured by a simple 1d SPM incorporating a single threshold. However, the rate coefficient and the threshold of incision trade off such that a given profile is not uniquely explained. To further build on this work we explore this problem using a suite of 1d and 2d models incorporating spatial patterns in climate, discharge records, and using thresholds of boulder mobility estimated in the field to test whether a single threshold can represent both initiation and cessation of river incision in this landscape.