Description: From McGimsey and others (2014): "In late August 2011, AVO was alerted by Troy Hamon, Chief of Resource Management, Katmai National Park, that the larger of two maar crater lakes on the caldera floor of Aniakchak Volcano had recently drained, leaving a gaping notch in the crater rim. The flood had ravaged the downstream drainage and alluvial fan as well as the upper reaches of the Aniakchak River, which drains Surprise Lake and the eastern caldera floor through The Gates. During September 8-9, 2011, authors Tina Neal and Game McGimsey, accompanied by Troy Hamon (on September 8), visited the site to document and study the activity.
"Aniakchak caldera formed about 3,600 years ago in a colossal eruption (Neal and others, 2001). Subsequent eruptive activity has occurred from many intracaldera vents, including eruptions that formed two maar craters, subsequently filling with water, that are situated on the southeastern caldera floor between the central Vent Mountain cone and the sheer inner east caldera wall. The larger maar is about 450 m (1,476 ft) by 515 m (1,690 ft), as measured rim to rim in Google EarthTM imagery (fig. 3); the smaller maar is 170 m (558 ft) in diameter at the rim. The maars formed subsequent to the draining of a caldera-wide lake, at least 1,000 years before present (BP), but they may be much younger, perhaps less than 400 years old.
"Geological investigations in 1992 documented that a low point on the maar rim was about 10-15 m (33-49 ft) above the lake surface, although higher strand lines were visible in the deposits surrounding the maar, one of which was located nearly at the low point (Neal and others, 2001). On the outboard flank of the maar-rim low-point, a seep - piping through the maar crater wall deposits to the lake - occurred near the base of the flank and formed the headwater for the stream that courses between a prominent lava flow from Vent Mountain and the colluvial slopes extending out from the eastern caldera wall. This stream flows about 1.7 km (1 mi) northward from its source at the maar to the Aniakchak River immediately inboard of The Gates, a prominent notch in the caldera wall through which the river drains the entire caldera.
"The maar breakout flood scoured and excavated the channel and banks of the stream north of the maar, cutting deeply into the colluvial slopes on the eastern side of the drainage. The previously well-vegetated alluvial fan near The Gates was entirely covered by flood deposits up to 1.5 m (4.9 ft) thick. Clumps of vegetation, up to about 1 m (3.3 ft) in length, were strewn on and within the deposit. The Aniakchak River was pressed even more tightly against the northern wall opposite the fan, causing some bank erosion, and the downstream river channel, sand bars, and banks were noticeably disrupted for more than 20 km (12.4 mi) beyond The Gates. The area of the breach on the maar rim was dramatically altered as downcutting progressed until a new base level was established. Discharge through the outflow at the northern end of the maar now approximates inflow at the southern end (primarily at the waterfall.
"Field measurements and observations indicate that the maar lake level declined 4.65 m (15.3 ft) from its recent high stand (determined from photographs taken July 18, 2010), as well as measurements made in the field in September 2011); this high stand was below the low-point on the northern rim where the breach occurred. The volume of water lost was at least 645,000 m3 (844,000 cubic yards), but this represents a minimum volume because there is no way of knowing how much additional water flooded into the maar immediately prior to initiation of the breakout flooding.
"Based on analysis of photographs, weather data, and observations from a local guide, the maar flood likely occurred between late July and late September 2010. Evidence of recent flooding on the southern floor and wall of the caldera, which drain into the maar, suggests that the maar flood was likely caused by the rapid and voluminous influx of water into the maar lake during a period of unusually heavy rainfall. The lake level rose rapidly and the low point on the maar rim may have been reached and spilled over. The increased hydraulic head may have simultaneously initiated failure of the crater wall at that location where groundwater seepage and possibly piping within the mantling deposit was occurring. The heavy precipitation may have further increased saturation of the deposit. Once the breach began, by either rapid downcutting, or structural failure of the wall, or both, rapid release of water occurred, resulting in a short-lived but massive flood down the drainage to - and then down - the Aniakchak River."