The modern Icelandic landscape: a record of regional uplift from progressive fluvial erosion histories

Iceland's dramatic landscapes provide an opportunity to disentangle histories of uplift generated by plate spreading, volcanism, dynamic support, and postglacial rebound. In these actively deforming regions fluvial systems are strongly regulated by uplift, and rapidly evolving river geometries record histories of vertical motions that can be used to examine the driving forces generating topographic relief. Broad knickzones observed along Iceland's large rivers, and its powerful waterfalls and deep canyons, hint that regional processes have generated significant relief. We constrain how Dettifoss, Europe's most powerful waterfall, evolved during the Holocene. Using drone photogrammetry, we map the 5 km long Jökulsárgljúfur waterfall system in Iceland and its enclosing canyon at cm-scale accuracy. We combine the resultant digital elevation model with cosmogenic ³He dating of fluvial terraces. Progressive younging of terraces indicates knickzone propagation rates up to ∼70 cm yr^-1 during the last 8 ka, which precludes megaflooding as the main erosional mechanism. A calibrated inversion of Iceland's drainage networks and isostatic calculations indicate that rifting, sub-plate support, and isostatic adjustment resulted in tens to hundreds of meters of regional Holocene uplift. Our results suggest that fluvial processes and knickzone migration set the pace of erosion in postglacial landscapes and that fluvial erosion can rapidly generate hundreds of meters of relief in post-glacial landscapes.