Showing papers by "Dimitri Lague published in 2014"

The stream power river incision model: evidence, theory and beyond

Abstract: The stream power incision model (SPIM) is a cornerstone of quantitative geomorphology. It states that river incision rate is the product of drainage area and channel slope raised to the power exponents m and n, respectively. It is widely used to predict patterns of deformation from channel long profile inversion or to model knickpoint migration and landscape evolution. Numerous studies have attempted to test its applicability with mixed results prompting the question of its validity. This paper synthesizes these results, highlights the SPIM deficiencies, and offers new insights into the role of incision thresholds and channel width. By reviewing quantitative data on incising rivers, I first propose six sets of field evidence that any long-term incision model should be able to predict. This analysis highlights several inconsistencies of the standard SPIM. Next, I discuss the methods used to construct physics-based long-term incision laws. I demonstrate that all published incising river datasets away from knickpoints or knickzones are in a regime dominated by threshold effects requiring an explicit upscaling of flood stochasticity neglected in the standard SPIM and other incision models. Using threshold-stochastic simulations with dynamic width, I document the existence of composite transient dynamics where knickpoint propagation locally obeys a linear SPIM (n=1) while other part of the river obey a non-linear SPIM (n>1). The threshold-stochastic SPIM resolves some inconsistencies of the standard SPIM and matches steady-state field evidence when width is not sensitive to incision rate. However it fails to predict the scaling of slope with incision rate for cases where width decreases with incision rate. Recent proposed models of dynamic width cannot resolve these deficiencies. An explicit upscaling of sediment flux and threshold-stochastic effects combined with dynamic width should take us beyond the SPIM which is shown here to have a narrow range of validity.

Erosion during Extreme Flood Events Dominates Holocene Canyon Evolution in North-East Iceland

Abstract: The importance of high-magnitude, short-lived flood events in controlling the evolution of bedrock landscapes is not well understood. During such events, erosion processes can shift from one regime to another upon the passing of thresholds, resulting in abrupt landscape changes that can have an important long lasting legacy on landscape morphology.Here we use topographic analysis and cosmogenic 3He surface exposure dating of fluvially sculpted surfaces to determine the timing of extreme flood events within the Jokulsargljufur canyon (North-East Iceland) and to constrain the mechanisms of bedrock erosion during these events. Once a threshold flow depth has been exceeded, the dominant erosion mechanism becomes the toppling and transportation of basalt lava columns and erosion occurs through the upstream migration of knickpoints. Surface exposure ages allow identification of three catastrophic erosive flood events about 9, 5 and 2 ka ago when multiple active knickpoints retreated large distances (> 2 km). Despite sustained high discharge of sediment-rich glacial meltwater (ranging from 100 to 500 m3 s-1), there is no evidence for a transition to an abrasion-dominated erosion regime since the last erosive flood event: the vertical knickpoints have not diffused over time and there is no evidence of incision into the canyon floor.We hypothesise that the erosive signature of the extreme events is maintained in this landscape due to the nature of the bedrock, the large scale of the river, large knickpoints and associated plunge pools, and the lack of transported coarse sediment (greater than gravel size). We explore these controls with an experimental study to define the possible influence of the following parameters on the dynamics of knickpoint migration and morphology in a controlled environment: discharge, flow regime, knickpoint height and initial topographic slope.