Description: From Bacon and others (2014): "Surprise Lake (1,055 ft, 322 m asl) is a remnant of a larger caldera lake evidenced by wave-cut benches on the northeast caldera walls and patches of lake sediment at several locations within the caldera (McGimsey and others, 1994). The lake filled until it overtopped a low point on the east rim of the caldera and then rapidly drained, possibly aided by eruptive activity (McGimsey and others, 1994) or by headward erosion of the canyon of the Aniakchak River. The draining produced a catastrophic flood as the ensuing torrent cut the gorge called The Gates in altered and fractured sedimentary rock (Waythomas and others, 1996). At the time of these publications, the ~1,610 ft (490 m) asl wave-cut terrace recognized by McGimsey and others (1994) was thought to represent the high stand of the caldera lake. Subsequent fieldwork indicated that all four dacite domes were emplaced subaqueously. Because West Dome has a summit elevation of 2,002 ft (610 m), the lake must have reached at least that height, although uplift or tilting of the southwestern caldera floor since the lake drained cannot be ruled out. McGimsey and others (1994) noted that Birthday Pass, elevation ~2,030 ft (620 m), evidently was not overtopped by the lake. Two additional low points in the caldera rim, northeast and east of Surprise Lake, are at ~2,130 ft (650 m) elevation. Both are underlain by Aniakchak volcanic rocks. Before downcutting, the minimum elevation of the caldera wall at The Gates implied by the topographic map (U.S. Geological Survey, 1963; fig. 10B [in original text]) could have been between ~1,900 ft (580 m) and ~2,100 ft (640 m). Because West Dome was emplaced beneath water, its summit elevation and the elevations of Birthday Pass and other low points on the caldera rim together suggest that the surface of the caldera lake probably reached close to 2,030 ft (620 m) asl before it overtopped the wall and cut The Gates. When the lake drained is constrained by radiocarbon ages of 1,850+/-40 yr B.P. on soil (silt) and 1,870+/-30 yr B.P. on wood from above pebbly sediment near the Aniakchak River mouth (VanderHoek and Myron, 2004, their table 8.1, appendix C, and p. 161-162). The weighted mean of these ages is 1,860+/-30 yr B.P. (table 1 [in original text]), which apparently dates a change from a pebbly beach to a depositional environment. The suggested date for the flood falls within an ~200 yr gap in human habitation near the mouth of the Aniakchak River (VanderHoek and Myron, 2004, p. 190-191). Evidence for lake sediments on apparently younger tuff cones within the caldera presented in the next section suggests that the catastrophic draining that produced the flood did not cut The Gates to their entire depth but left a residual lake that was larger and higher than present Surprise Lake."