Slackwater sediments refine the timing of Late Holocene Icelandic canyon-carving floods and confirm they were smaller than reported

Abstract

Catastrophic floods have formed deep bedrock canyons on Earth, but the relationship between discharge and erosion remains contested. This hinders efforts to use geological evidence of these cataclysmic events to constrain their magnitude – a prerequisite for impact assessments. This study combines evidence from slackwater sediments with topographic models and hydraulic simulations to constrain the Late Holocene flood history of the Jökulsá á Fjöllum river in northern Iceland. We date floods to 3.5, 1.5 and 1.35 cal. ka BP and confirm that discharge peaks during these events were much smaller than previously reported. In light of recent cosmogenic evidence that nearby knickpoints retreated over 2 km during these floods, our results strengthen simulations and observations indicating that the extent of bedrock erosion is not necessarily controlled by discharge. These findings support a growing consensus that the magnitude of canyon-carving floods is much smaller than typically assumed.