Great Holocene floods along Jokulsa a Fjollum, north Iceland

Abstract

Jökulsá á Fjöllum, Iceland's largest glacial river, drains from Vatnajökull icecap northward to the sea along a broad low that includes an active volcanic belt. Geomorphic features along this path reveal an ancient discharge of water large enough to fill the river valley and spill among a plexus of lows in the volcanic landscape. Stratigraphy in most places reveals just one late Holocene great flood down Jökulsá á Fjöllum, between 2500 and 2000 yr ago. Step-backwater computation suggests its peak flow was 0.7 million m 3 /s or more. An early scabland-carving great flood had swept down the Ásbyrgi area of lowermost Jökulsá just after deglaciation, 9000–8000 yr ago. Stratigraphy near Vesturdalur reveals at least 16 additional floods, perhaps of moderate discharge, between about 8000 and 4000 yr ago. Dispersed field evidence of the late Holocene great flood–anastomosing channels whose basalt surfaces are water fluted and half-potholed, in places plucked down to small-scale scabland replete with dry cataracts, huge boulders, long gravel bars, giant current dunes—is traced the length of Jökulsá valley. From Vatnajökull's north margin at Kverkfjöll, water anastomosed through diverse lows of a high-relief landscape. Thus swift release of meltwater from subglacial Kverkfjöll caldera must have been a source of flood. But even this catastrophic outflow was insufficient to constitute the huge discharges evident farther downvalley. Field evidence reveals a yet greater discharge directly from the large outlet glacier Dyngjujökull. There is no evidence that subglacial Bárðarbunga caldera was involved, but subglacial melting during eruption of a more eastern fissure system could be a source of flood.