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

• Published in: Proceedings of the National Academy of Sciences - PNAS 2015-02, Vol.112 (8), p.2355-2360
• Main Authors: Baynes, Edwin R. C., Attal, Mikaël, Niedermann, Samuel, Kirstein, Linda A., Dugmore, Andrew J., Naylor, Mark
• Format: Article
• Language: English
• Subjects: bedrock; Evolution; Floods; Iceland; landscapes; Morphology; Physical Sciences; Soil erosion; Topography
• ISSN: 0027-8424; 1091-6490
• DOI: 10.1073/pnas.1415443112

Significance The importance of high-magnitude, short-lived events in controlling the evolution of landscapes is not well understood. This matters because during such events, erosion processes can surpass thresholds and cause abrupt landscape changes that have a long-lasting legacy for landscape morphology. We show that extreme flood events, during which the flow depth exceeds the threshold for erosion through plucking rather than abrasion, are the dominant control on the evolution of a large bedrock canyon in Iceland. The erosive signature of these events is maintained within a dynamic landscape over millennial timescales, emphasizing the importance of episodic extreme events in shaping landscapes. Extreme flood events have the potential to cause catastrophic landscape change in short periods of time (10 ⁰ to 10 ³ h). However, their impacts are rarely considered in studies of long-term landscape evolution (>10 ³ y), because the mechanisms of erosion during such floods are poorly constrained. Here we use topographic analysis and cosmogenic ³He surface exposure dating of fluvially sculpted surfaces to determine the impact of extreme flood events within the Jëökulsëöárgljëöáúfur canyon (northeast Iceland) and to constrain the mechanisms of bedrock erosion during these events. Surface exposure ages allow identification of three periods of intense canyon cutting about 9 ka ago, 5 ka ago, and 2 ka ago during which multiple large knickpoints retreated large distances (>2 km). During these events, a threshold flow depth was exceeded, leading to the toppling and transportation of basalt lava columns. Despite continuing and comparatively large-scale (500 m ³/s) discharge of sediment-rich glacial meltwater, there is no evidence for a transition to an abrasion-dominated erosion regime since the last erosive event because the vertical knickpoints have not diffused over time. We provide a model for the evolution of the Jëöáúökulsëöáúöárgljëöáúöáúfur canyon through the reconstruction of the river profile and canyon morphology at different stages over the last 9 ka and highlight the dominant role played by extreme flood events in the shaping of this landscape during the Holocene.