Late Holocene canyon-carving floods in northern Iceland were smaller than previously reported

Willem G M Van Der Bilt,Iestyn D Barr,Kathryn Adamson,Jostein Bakke,Rick Hennekam,Timothy Lane,Sarah M P Berben

doi:10.1038/s43247-021-00152-4

Abstract

Catastrophic floods have formed deep bedrock canyons on Earth, but the relationship between peak discharge and bedrock erosion is not clearly understood. This hinders efforts to use geological evidence of these cataclysmic events to constrain their magnitude – a prerequisite for impact assessments. Here, we combine proxy 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 thousand years ago and confirm that flow peaks during these events were at most a third of previous estimates. Nevertheless, exposure ages suggests that nearby knickpoints retreated by more than 2 km during these floods. These findings support a growing consensus that the extent of bedrock erosion is not necessarily controlled by discharge and that canyon-carving floods may be smaller than typically assumed.

Highlights

Catastrophic floods have formed deep bedrock canyons on Earth, but the relationship between peak discharge and bedrock erosion is not clearly understood
Jökulsá á Fjöllum Jökulsá á Fjöllum watershed Rift zone Reykjanes Volcanic Belt (RVB) Western Volcanic Zone (WVZ) Bárðarbunga Askja Krafla Grímsvötn Kverkfjöll Hekla Katla calibrated hydraulic models can be parameterized with centimeter-scale Digital Elevation Models (DEMs) to produce highly accurate peak discharge estimates[11]
Investigation of a processed Ground Penetrating Radar (GPR) profile across our coring site reveals a sharp facies change from continuous reflectors to reflection-free at 5 m sediment depth (Fig. 2D)

Summary

Introduction

Catastrophic floods have formed deep bedrock canyons on Earth, but the relationship between peak discharge and bedrock erosion is not clearly understood. This study harnesses the potential of these advances to deepen our understanding of the links between flood magnitude and bedrock erosion by targeting slackwater sediments deposited by the Jökulsá á Fjöllum river in northern Iceland (Fig. 1). Based on this sediment signature and its overlap with modern deposition, we argue that units 1 and 6 were laid down when the Ástjörn basin was occupied by a lake that did not receive clastic material from the river or sandur.

Results

Conclusion