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Jo??kulsa?? a?? Fjo??llum, Iceland's largest glacial river, drains from Vatnajo??kull 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 Jo??kulsa?? a?? Fjo??llum, between 2500 and 2000 yr ago. Step-back water computation suggests its peak flow was 0.7 million m3/s or more. An early scabland-carving great flood had swept down the A??sbyrgi area of lowermost Jo??kulsa?? 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 Jo??kulsa?? valley. From Vatnajo??kull's north margin at Kverkfjo??ll, water anastomosed through diverse lows of a high-relief landscape. Thus swift release of meltwater from subglacial Kverkfjo??ll caldera must have been a source of flood. But even this catastrophic outflow was insufficient to constitute the huge discharges evident farther down-valley. Field evidence reveals a yet greater discharge directly from the large outlet glacier Dyngjujo??kull. There is no evidence that subglacial Ba??ra??oarbunga caldera was involved, but subglacial melting during eruption of a more eastern fissure system could be a source of flood.
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Great Holocene floods along Jo??kulsa?? a?? Fjo??llum, north Iceland
International Association of Sedimentologists Special Publications