Progressive Growth, Modulated Supply: How coupling and decoupling between an enormous retrogressive thaw slump and its depositional fan impacts sediment delivery to the Selawik River, Northwest Alaska

Permafrost degradation throughout the Arctic is resulting in an increased frequency of melt-driven hillslope instability. Retrogressive thaw slumps (where a steep headwall retreats upslope into massive, ice-rich deposits) deliver enormous fluxes of fine and coarse grain sediment to downslope bodies of water. As anomalous warming persists in the Arctic, the initiation and persistence of these features is anticipated to increase. In 2004, in the Selawik River in Northwest Alaska, a retrogressive thaw slump initiated above a tall, steep cut bank and has since propagated upslope to become the largest feature of its kind know in North America (~220m x 200m and 25m deep). In the 5 years since it was first identified in 2004, 540,000 cubic meters of ice and sediment have been eroded and were either deposited in a river-damming fan (184,000 cubic meters) or fluxed into the stream (356,000 cubic meters). Between two high resolution topographic surveys completed in 2007 and 2009, the headwall has retreated ~35 meters, eroding a volume of 187,000 cubic meters in two years. The fate of this massive flux of meltwater and sediment is determined by storage either in the floor of the slump or on the fan at its mouth. Field observations suggest that the floor and the fan follow a cyclic pattern of cut and fill modulated by three processes (1) the rate of sediment production at the headwall, (2) the rate of river erosion at the fan toe and (3) the critical gradient necessary to flux material across the slump floor and fan surface. Optical sensors downstream of the slump record a 10 fold increase in turbidity during periods of stable weather and greater values during storms. Because of the fine grain nature of the sediments, the river remains heavily clouded with grey silt for over 100 km downstream. Sheefish spawning habitat located downstream of the slump are affected by fine grain sedimentation during low flow periods, but during high flow events, fines are suspended and swept from the system. Monitoring of slump morphology, stream geometry and water quality over the next 3 years will further quantify how sediment flux is temporally modulated by the interactions between the slump, the fan and the river.