Retrogressive thaw slumps (RTS) are dramatic slope failures caused by thaw of ice-rich permafrost. In a RTS, an escarpment at the upper end of the slump advances by thaw, sediment falls onto the adjacent slump floor, and then is transported away by water erosion or sliding. Our comprehensive inventory of RTS in the 5 National Park Service units of northern Alaska (82,000 km2) located over 700 RTS that exposed a total of about 2.4 km2 of bare soil. These RTS are mostly in valleys of the Brooks Range that were glaciated in the Late Plestocene, and not in the ice-rich eolian (yedoma) deposits of unglaciated areas like the northern Seward Peninsula. Escarpments revealed glacial ice 1 to 4 m below the surface in many slumps. Examination of historical aerial photographs from the c. 1950 and c. 1980 show that RTS were also active in the past, though probably not as active as today. Some current RTS are re-working areas covered by RTS in the past, indicating that residual ice remained after previous slumping episodes. NPS is monitoring the growth of selected slumps with 3D models created from aerial photographs. Scarp heights ranges from 1.5 to 20 m. Scarp retreat rates of over 20 m per year have been observed on several slumps. Differencing the elevations models between years showed that annual subsidence in the newly formed part of the slumps was comparable to the scarp height. On most of the slumps, liquefied material adjacent to the scarp flowed downhill into an accumulation zone coinciding approximately to the zone of the previous year's subsidence, where the surface rose a meter or two.
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