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 Jökulsárgljúfur 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.