Catastrophic impact of extreme flood events on the morphology and evolution of the lower Jökulsá á Fjöllum (northeast Iceland) during the Holocene

The impact of extreme flood events is rarely considered in studies of long-term landscape evolution, despite the potential for catastrophic landscape change in a short period of time. Here, we use an integrated approach of geomorphological mapping, topographic analysis and geophysical surveys to identify and quantify the impact of extreme flood events (jökulhlaups) along the Jökulsá á Fjöllum, Iceland, where evidence for the action of such floods is widespread on microspatial to macrospatial scales. The apex of the 28-km-long Jökulsárgljúfur canyon is characterised by a complex network of palaeo-flood channels and large vertical knickpoints such as Dettifoss (54m high) and Hafragilsfoss (20m high). Downstream, the Forvoð valley contains large terraces of boulder-rich deposits (50m thick, >3km long). Near the outlet of the canyon is Ásbyrgi, a dry canyon (3km long, 1km wide, up to 90m deep) with eroded cataracts and scabland morphology immediately upstream and ~90m above the current river channel. Topographic analysis and electrical resistivity tomography surveys show that 0.144km3 of rock was eroded from Ásbyrgi during its formation ~10,000years ago, and just 4% of this eroded volume is currently filled with sediment deposits, up to 5m thick. Deposited boulders across the canyon floor of Ásbyrgi demonstrate that the discharge of the jökulhlaup that formed the canyon was at least 39,000m3s−1. We present a model for the evolution of the lower Jökulsá á Fjöllum and the Jökulsárgljúfur canyon during various stages of an extreme flood event. Reconstruction of the early Holocene flood event includes the initiation and development of different canyons before the capture of all floodwater within one canyon at the end. We tie the evolution of the lower Jökulsárgljúfur canyon to established chronology of flood events during the Holocene farther upstream and highlight the dominant impact of extreme flood events over background processes in this landscape.