The Alaska Volcano Observatory (AVO) has an extensive collection of photographs and satellite images of volcanoes throughout Alaska contributed by AVO scientists and other observers. This database (Cameron and Snedigar, 2016) was consulted for representative photographs of the crater and caldera lakes described in this paper. Most of the photographs are relatively recent images of the lakes and thus show the near present configurations of the water bodies. Most of these photographs are shown in the Supplementary Figures that accompany this paper.

Spatial information about Alaska crater lakes is presented in the form of topographic maps and cross sections generated from 5 m-resolution ifSAR elevation data acquired 2012–2019 (Carswell, 2013). Digital elevation data for each crater lake was processed in Qgis where hillshade renderings and contour maps were constructed. The digital elevation data was used to identify the lowest point along the crater rim where spillover of lake water would likely occur, if the crater were filled to capacity. Topographic cross sections also were generated in Qgis through each crater rim low point; the cross sections provide a two-dimensional representation of each crater and are shown in the Supplementary Figures that accompany this paper. The maximum crater lake area was taken as the elevation contour corresponding to the lowest point on the crater rim. The maximum potential lake depth was approximated as the elevation difference between the crater rim low point and the crater floor elevation including the lake depth if known or estimated. The maximum crater lake area multiplied by the depth gives a maximum theoretical lake volume estimate (Table 1) which is considered an upper bound on lake volume as this estimate does not explicitly account for crater geometry. A second volume estimate was made by assuming a truncated cone for crater shape and using the formula, V = 1 3 π d [ ( r 2 + r ) ( R + R 2 ) ] to calculate maximum water volume (V), where r is bottom radius, R is top radius at the spillover elevation, and d is depth. In many instances the volume estimates are about the same (Table 1) but in a few cases (Douglas, Makushin) the values differ by more than a factor of 10. Photographs of these two crater lakes indicate that the crater shapes are better approximated by a truncated cone so the volume estimates resulting from application of the truncated cone formula are used for these two volcanoes in subsequent calculations described below. A third maximum potential lake volume estimate was made using the ifSAR digital elevation data and the raster surface volume algorithm in Qgis. This method does not include the present lake volume, but where known gives volume estimates roughly comparable to the estimates made by the other two methods (Table 1). In most cases, application of the formula for a truncated cone, where the lake bottom elevation is known or reasonably approximated, gives the plausible estimates for maximum potential lake volume because qualitatively, many of the craters look like truncated cones. In cases where modern lakes are absent or small in volume, the Qgis raster volume estimates of maximum potential lake volume were used for subsequent calculations of maximum peak discharge because these estimates are based on actual ifSAR derived topography. The values used for peak discharge estimates are shown in bold in Table 1.

Published geologic information was consulted to make first-order evaluations of crater rim conditions to provide insight into crater wall geology, stability and susceptibility to erosion or failure. At many of the locations discussed, geologic information is limited or nonexistent. The specific reports and articles consulted are given as references throughout Crater Lakes in Alaska.

Volcanic crater rim stability and susceptibility to failure are discussed in Crater Stability and Susceptibility to Failure of this paper. To date, there have been no rigorous evaluations of the strength of lake-filled craters using any of the standard approaches for determining rock-mass strength and slope stability (Watters et al., 2000). A first-order attempt to evaluate the seismic effects on crater wall stability is presented in Seismic Effects on Volcanic Crater Lakes. To date there have been no systematic studies of Alaska crater lake hydrology although some general conclusions about the hydrologic characteristics of the lakes are possible. These are described further in Volcanic Crater Lake Hydrology.

The approach used to estimate maximum peak discharge associated with floods from lake-filled craters in Alaska is described in Crater Rim Overtopping, Breach Formation, and Peak Flood Discharge. This was done to benefit from the context established by the review of lake-filled volcanic craters in Alaska presented in Crater Lakes in Alaska and the analysis of crater rim stability and hydrology presented in Crater Stability and Susceptibility to Failure. The basic approach to dam-break analysis is described in Waythomas et al. (1996) but modified here for application to crater lakes.

Mount Spurr volcano is in the Cook Inlet region of south-central Alaska (Figure 1). The volcano consists of a stratified assemblage of andesite lava flows and lahar, pyroclastic-flow, and debris-avalanche deposits of late Quaternary age (Nye and Turner, 1990). These rock units are truncated by a horseshoe-shaped avalanche caldera of Holocene age that is open to the southeast (Supplementary Figure S1). The summit cone of Mount Spurr is heavily glaciated and rock outcrops on the upper flanks of the volcano are largely obscured by snow and ice. The volume of ice and perennial snow on Mount Spurr when measured in 1981 was 67 ± 17 km³ (March et al., 1997). A period of elevated heat flow at the summit in 2004–2006 resulted in partial exhumation of a crater on the summit edifice and formation of a small lake within the crater with an interior volume (Supplementary Figure S1; Coombs et al., 2006). By the end of this event, a crater about 9.7 × 10⁶ m³ was visible at the summit of the volcano. A small lake developed on the floor of the crater (first observed in 2004), covering an area of about 11,540 m² by 2006 (Supplementary Figure S1). The maximum volume of a hypothetical lake in the summit crater is 5.5–9.7 × 10⁶ m³ (Table 1). The thermal excursion of 2004–2006 is the only known period associated with the exhumation and formation of a crater lake at the Mount Spurr summit. Previous thermal events did occur that partially exposed the summit crater (Coombs et al., 2006) but no lakes were reported.

Crater Peak is a basaltic–andesite satellite cone on the south flank of Mount Spurr volcano (Supplementary Figure S2). Crater Peak has been the active vent of Mount Spurr volcano for about the past 5–6 ka (Riehle 1985) and was the site of the two documented historical eruptions in 1953 and 1992 (Keith, 1995). The first known historical eruption occurred on July 9, 1953 (Juhle and Coulter, 1955; Miller et al., 1998). It is unknown if a lake occupied the floor of Crater Peak prior to the 1953 eruption, but for several decades prior to the eruption, plumes of whitish steam commonly were observed rising from the summit of Crater Peak and pilots reported an increase in the vigor of steaming in late spring 1953 (Juhle and Coulter, 1955). The 1953 eruption was a single explosive burst lasting about 1 hour, and it generated an ash cloud that rose more than 10 km above sea level (Juhle and Coulter, 1955).

Aerial photographs taken in 1952, 1954, and 1957 show no lake on the floor of Crater Peak (Supplementary Figure S2). A 1970 visit to Crater Peak indicated a hot lake was present in the crater (Keith et al., 1995). A Landsat satellite image acquired on July 27, 1974, also shows no lake on the crater floor. An aerial photograph taken on August 8, 1978, shows a lake present on the crater floor (Supplementary Figure S2d) and opportunistic photographs in the AVO photo database show a lake present from 1982 until the first Crater Peak eruption in 1992 on June 27. Intermittent observations of Crater Peak from 1992 to the present have indicated that a lake has not reformed after the 1992 eruption.

The low point on the crater rim is about 1940 m above sea level and the depth from the low point to the crater floor is about 40 m. The area of the crater is about 360,450 m² and the maximum volume of a crater-filling lake is about 1.8–14 × 10⁶ m³ (Table 1). Observations and photographic evidence indicate that at no time was the lake ever much more than a shallow feature on the floor of Crater Peak covering an area of about 50,000 m².

Mount Douglas is located on the west side of the marine entrance to Cook Inlet (Figure 1). Douglas is an ice- and snow-covered stratovolcano (summit altitude 2,140 m) with a summit crater that contains a small lake (Supplementary Figure S3). Little is known about the geologic history of Douglas volcano, and it is uncertain when the last eruption of the volcano occurred and when the summit crater developed. Measurements made in 1991 indicated that the lake had a temperature of 25°C and a pH of ∼1 (Motyka et al., 1993). The summit crater has an area of about 134,000 m² and the crater lake covers an area of about 61,470 m² (average of four measurements made from satellite images, Table 1). The low point on the crater rim is about 10 m above lake level, and the maximum crater volume at this level is about 9.8–10 × 10⁵ m³ (Table 1).

Mount Chiginagak is an ice and snow-clad stratovolcano on the Alaska Peninsula near Mother Goose Lake (Figure 1). The volcano is about 8 km in diameter and its summit has an elevation of 2,135 m above sea level. The summit crater contains a lake (Supplementary Figure S4) that episodically drains through a glaciated breach on the crater rim (Schaefer et al., 2008). The volcano has had no known historical eruptive activity, although fumaroles on the upper flanks of the edifice are occasionally quite vigorous. The volcano developed during the late Quaternary and lava flows, lahar, pyroclastic, tephra and debris-avalanche deposits of Holocene and Pleistocene age characterize the edifice (Schaefer et al., 2017). Rocks in the summit area are hydrothermally altered although the type and extent are not known (Supplementary Figure S4; Schaefer et al., 2017).

The summit crater of Chiginagak volcano has an area of about 215,610 m² although a pronounced glacial notch in the rim (Supplementary Figure S4) precludes the entire crater becoming filled with water. The crater lake is an ephemeral feature that periodically develops when the heat flux through the upper edifice increases to melt snow and ice in the crater. As the lake level rises, the water encounters a glacier-occupied saddle (Supplementary Figure S4) that is the low point on the crater rim. At this elevation the maximum area a crater lake could attain is about 73,400 m² and the hypothetical lake volume is 1.3–3.8 × 10⁶ m³ (Table 1). Erosion of the ice dam allows, acidic lake water to flow from the crater and enter the Indecision Creek drainage on the west-northwest flank of the volcano (Schaefer et al., 2008). Acidic water floods are temporarily destructive to the riparian ecosystem of the Indecision Creek drainage including Mother Goose Lake (Schaefer et al., 2008).

Kaguyak volcano and its defining caldera lake is a silicic volcanic center consisting of numerous nested lava domes and flows. The volcano is in Katmai National Park on the northeastern Alaska Peninsula (Figure 1). Kaguyak experienced a caldera-forming eruption about 5.8 ka (Fierstein, and Hildreth, 2008) and a lake formed in the small 2.5 × 3 km diameter caldera after the eruption. The lake is estimated to be about 150 m deep, has a surface area of 3.6 × 10⁶ m² and an approximate volume of 5 × 10⁸ m³ (Fierstein and Hildreth, 2008). The caldera rim has a distinct low point where the lake spillover should occur if the water level increased by about 30 m (Figure 2). The maximum water volume the caldera could hold is 1.3–9 x 10⁸ m³ (Table 1). Information about modern lake level fluctuations is not known.

Kaguyak has had no known historical activity and the exposed walls of the caldera do not appear significantly altered. At least three nested dome complexes were identified by Fierstein and Hildreth (2008) that make up the bulk of the volcano (Figure 2). There are no outward signs of caldera rim instability although areas of edifice overlap could be somewhat less structurally robust than other parts of the caldera rim. The low point on the caldera rim (Figure 2B) is underlain by a block-rich breccia emplaced about 6–6.5 ka, that was associated with a debris-avalanche produced by dome disruption that preceded caldera collapse (Fierstein, and Hildreth, 2008). The “Big Breccia” deposit fills preexisting drainages and low areas and is one of the main deposits that holds in the lake (Fierstein, and Hildreth, 2008). The deposit is massive, unstratified, about 25 m thick, and contains numerous blocks 2–4 m in diameter. The deposit matrix consists of coarse sandy material that is iron-stained and somewhat indurated possibly as a result of iron cementation (Fierstein and Hildreth, 2008).

Mount Katmai is a compound stratovolcano, about 10 km in diameter, that is also located in Katmai National Park on the northeastern Alaska Peninsula (Figure 1). Mount Katmai experienced catastrophic caldera collapse during the June 1912 Katmai-Novarupta eruption (Hildreth and Fierstein, 2012a). The caldera has an area of 9.4 × 10⁶ m² and is partially occupied by a lake of about 5.4 × 10⁶ m² in area. The low point on the caldera rim is at an elevation of 1,470 m and the lake surface has an elevation of about 1,285 m (Supplementary Figure S5). The estimated elevation of the caldera floor is about 995 m, indicating that the lake has a depth of 290 m (Hildreth and Fierstein, 2012a). The maximum depth of a crater-filling lake is about 470 m and the maximum volume of water the crater could hold is 1.5–3.5 × 10⁹ m³ (Table 1). Upwelling associated with hydrothermal activity on the floor of the lake was reported by Motyka (1978).

Multiple glaciers that resided on the Mount Katmai edifice were truncated by caldera collapse in 1912 (Hildreth and Fierstein, 2012a). Several of these glaciers persist today and occupy shallow glacial troughs on the edifice flanks. Runoff from snow and ice melt and precipitation probably controls the water balance of the lake as there are no perennial streams that flow into the caldera.

The rocks exposed in the walls of the caldera consist of bedded lavas of basalt to rhyodacite composition and a variety of phreatomagmatic fragmental deposits (Hildreth and Fierstein, 2012b). Alteration is common on parts of the caldera wall (Supplementary Figure S5) and the extent of alteration is locally intense enough to obscure primary depositional features such as along the east wall of the caldera where the caldera rim is at its lowest elevation (Figure 6 of Hildreth and Fierstein, 2012b). A possible zone of weakness on the northeast wall of the caldera is indicated by numerous dikes oriented roughly northeast-southwest (Hildreth and Fierstein, 2003; Hildreth and Fierstein, 2012b).

Mount Mageik is one of the Katmai group volcanoes located in Katmai National Park in the northeastern part of the Alaska Peninsula (Figure 1). The volcano consists of a 2,165-m-high compound andesite-dacite stratovolcano that includes four overlapping cones that make up a broad edifice that covers about 80 km² (Hildreth et al., 2000). A small, elongated crater about 350 m in length on the northeast side of the central peak contains a shallow crater lake that is also the source of vigorous fumarolic activity (Supplementary Figure S6). The crater lake covers an area of about 8,600 m² and the crater is roughly 8.5 × 10⁴ m² in area. The low point on the south crater rim is about 1925 m above sea level (Supplementary Figure S6). At this point the maximum potential depth of a crater-filling lake is about 35 m and the corresponding lake volume would be about 3–22 × 10⁵ m³ (Table 1). The crater resides in andesite-dacite lava flows of late Pleistocene–Holocene age (Hildreth and Fierstein, 2003). Photographs of the crater indicate extensive areas of hydrothermal alteration (Supplementary Figure S6) indicating that the crater walls could be susceptible to failure or erosion by water.

Mount Martin is a small volcanic cone of Holocene age that consists of numerous overlapping lava flows of blocky dacite (Hildreth and Fierstein, 2003). The volcanic pile rests on a high ridge of Jurassic rocks that provides much of the relief of the volcano. Martin has an active hydrothermal system and numerous jetting fumaroles along the floor of its crater just beyond the margin of a small crater lake (Supplementary Figure S7). The Martin crater has an area of about 1.2 × 10⁵ m², and the lake covers about 9,700 m². The maximum lake volume the crater could hold is about 1–5 × 10⁵ m³ (Table 1). The cone of Mount Martin consists primarily of andesite ejecta including scoria, agglutinate, and phreatomagmatic breccia (Hildreth and Fierstein, 2003). These deposits are exposed in the crater walls and exhibit variable alteration.

Mount Dana is a small volcanic center with a central crater about 2 km in diameter, located northeast of Pavlof Bay on the southwestern Alaska Peninsula (Figure 1). Little is known about the geology of the volcano and its eruptive history. Pyroclastic-flow deposits emplaced about 3.8 ka and presumably associated with a large explosive eruption that could have formed the crater are exposed on the west flank of the edifice (Wilson et al., 1995).

The summit crater of Dana volcano has an area of about 2.7 × 10⁶ m², and the crater lake covers about 5.8 × 10⁵ m². Because the crater is breached on the southwest flank and there is an outlet stream that exiting the lake (Supplementary Figure S8), it is unlikely that the lake could get much larger unless the outlet became blocked by a landslide. The depth of the lake is unknown, but is probably on the order of 30–40 m based on visual estimates made during a field visit by the author in 2005. For a lake 40 m deep, the lake volume would be 2.3–2.6 × 10⁷ m³ (Table 1).

Mount Hague is a prominent stratocone within Emmons Lake caldera on the southwestern Alaska Peninsula (Figure 1). Mount Hague consists of a twin-peaked basaltic-andesite to dacite stratovolcano about 5 km wide at the base and about 750 m high (Supplementary Figure S9). The flanks are largely snow- and ice-covered and scalloped by glacial erosion. The summit area is marked by two over-lapping circular craters, each about 400 m in diameter (Supplementary Figure S9). The rim of the northern crater reaches an elevation of 1,540 m above sea level but is cut by the southern crater, indicating the latter is younger. The southern crater is about 250 m deep, and a fumarole field is visible when the crater floor is exposed; however, by mid-summer the bottom of the crater is commonly covered by an ephemeral lake (Supplementary Figure S9). The low point on the crater rim is about 1,350 m and the crater floor is about 1,260 m, indicating a maximum potential lake depth of about 90 m. For a lake of this depth, the maximum volume is about 4.6–13 × 10⁶ m³ (Table 1).

Makushin Volcano is a 2,036-m-high stratovolcano on Unalaska Island located 8 km west of Dutch Harbor and Unalaska, Alaska (Figure 1). The volcano has had at least 17 eruptive episodes since the 1700 s and erupted most recently in 1995 (McGimsey and Neal, 1995; Miller et al., 1998). The summit crater has an area of about 350,000 m² and is roughly 217 by 250 m in diameter (Supplementary Figure S10). The lake on the floor of the crater has an area of about 24,500 m² (Supplementary Figure S10). The crater was the source of minor eruptive activity in 1995 when a single eruptive burst generated an ash cloud that reached about 2,400 m above sea level and produced trace ash fallout on the upper part of the edifice (McGimsey and Neal, 1996).

The summit crater is surrounded by an extensive snow and ice field and there are multiple active fumaroles on the crater floor (Werner et al., 2020). The hydrothermal system at Makushin is active and likely driven by shallow magma beneath the summit region (Motyka, et al., 1988). Rock and pyroclastic deposits exposed in the walls of the crater are hydrothermally altered, indicating that the rock mass strength of the crater is likely compromised and could be susceptible to erosion or failure should a larger lake develop in the crater. The crater has a depth of about 100 m and thus the maximum volume of water the crater could hold is 4.2–19 × 10⁵ m³ (Table 1).

Tana volcano forms the eastern part of Chuginadak Island and is located east of Cleveland volcano (Figure 1). Little is known about the geology and eruptive history of the volcano. Photographs of the crater (Supplementary Figure S11) and studies of surficial deposits on the flanks of Tana (Persico et al., 2019) indicate that parts of the edifice are hydrothermally altered. Hot springs and active fumaroles on the flanks of the volcano are described in Werner et al. (2020). The crater on Tana has an area of about 7.7 × 10⁵ m², and the crater lake covers about 2.1 × 10⁵ m². The low point on the crater rim is about 780 m, only about 10 m above the modern lake level. The maximum possible volume of a crater lake is about 0.5–3.4 × 10⁶ m³ (Table 1).

Herbert volcano on Herbert Island, is in the Islands of the Four Mountains area of the Aleutian Islands and is the western most volcano of this group of volcanic islands (Figure 1). Little is known about the geology and eruptive history of Herbert. The summit of the volcano is characterized by a circular 2.1 km diameter crater (or small caldera) that hosts a small lake that is about 5.7 × 10⁵ m² in area (Supplementary Figure S12). The low point on the crater rim is about 840 m and this corresponds to a maximum potential lake area of about 1.9 × 10⁶ m². The depth of the largest lake the crater could hold is about 220 m and the volume 1.8–4.2 × 10⁸ m³ (Table 1).

Korovin is one of several stratovolcanoes on the northern part of Atka Island in the central Aleutian Islands (Figure 1). The village of Atka is 21 km south of the volcano. The Korovin summit edifice includes two craters, and the southern crater hosts the active vent and contains a small warm lake. Korovin has had minor episodes of phreatomagmatic eruptive activity in 2005, 1998, 1996, 1987, 1986, 1976, 1973, 1954, 1951, and 1907. Some of these eruptions generated small amounts of ash and lava flows of limited extent (Miller et al., 1998; McGimsey et al., 2003). The 1998 eruption produced an ash cloud that reached about 9 km above sea level. Fumaroles and warm mud springs are plentiful on the western and southern flanks of the volcano and overall the level of hydrothermal activity on and around the Korovin edifice is relatively high.

The south crater of the Korovin edifice has an area of about 3.7 × 10⁷ m² and is partially occupied by a lake with an area of 1.6 × 10⁴ m² (Supplementary Figure S13). The maximum depth of a crater filling lake is about 140 m and the maximum volume 2.3–3.6 × 10⁷ m³ (Table 1). The walls of the south crater consist of interbedded lava flows and pyroclastic deposits that do not appear significantly altered except at the base of the crater around the crater lake (Supplementary Figure S13).

Kasatochi is a small, isolated island volcano in the east-central part of the Aleutian Arc (Figure 1). The volcano makes up all of Kasatochi Island and consists of a circular cone about 3 km in diameter with a lake-filled central crater (Supplementary Figure S14) that exhibited minor fumarolic activity in 2005 (McGimsey et al., 2008). Kasatochi experienced a 21-hour-long VEI 4 eruption on August 7–8, 2008 that was the only known historical eruption of the volcano (Waythomas et al., 2010a). As a result of the 2008 eruption, the summit crater was widened by several tens of meters and Kasatochi Island was covered with many tens of meters of pyroclastic debris and fine ash (Nye et al., 2017; Scott et al., 2010; Waythomas et al., 2010b). The Kasatochi summit crater has an area of about 1.5 × 10⁶ m², and the crater lake has an area of 8 × 10⁵ m² as determined from a November 22, 2020 WorldView-3, satellite image (Table 1). The crater lake had a depth of about 20 m when estimated in 2009 (Nye et al., 2017). The low point on the crater rim is about 160 m above sea level indicating that the maximum depth of the largest possible crater lake is roughly 180 m (Supplementary Figure S14). The area of the largest possible lake in the crater is about 1.3 × 10⁶ m² and the volume about 2.2 × 10⁸ m³.

Geologic studies of Kasatochi following its 2008 eruption indicate that the walls of the crater consist of variable assemblages of bedded lavas and pyroclastic deposits (Figure 7, Supplementary Figure S14). The pyroclastic deposits on the west and south rims of the crater are loose and not well consolidated and are likely susceptible to erosion by water. The lava flows exposed beneath the low point on the crater rim are thin bedded with rubbly margins (Nye et al., 2017) and could be susceptible to piping should the crater lake increase in volume and approach the low point on the crater rim. Conduit-like, focused flow of water into the crater after the 2008 eruption (Waythomas et al., 2010a) indicates that parts of the crater walls could allow piping style drainage.

The north crater of Cerberus volcano on Semisopochnoi Island (Figure 1) contains an ephemeral lake that was observed during recent periods of unrest in 2020-2021 at north crater. The crater is about 400 m in diameter and has an area of 1.5 × 10⁷ m². Intermittent satellite observations since 2014, indicate that a small ephemeral lake is sometimes present on the crater floor and likely develops in response to snowmelt runoff sometimes induced by unrest related geothermal heat flow and fumarolic activity within the upper edifice of the north cone. Although its size varies, the crater lake has an area of 6–9 × 10³ m³. The low point on the crater rim is about 750 m above sea level, and at this elevation the maximum area of a crater lake is 1 × 10⁵ m². The maximum depth is about 120 m and thus the greatest volume of water the crater could hold is about 6–7 × 10⁶ m³ (Table 1).

The north cone is a basaltic-andesite edifice that consists of lava flows and pyroclastic deposits which are exposed in the walls of the crater (Coombs et al., 2018). None of these deposits appear altered, and the crater itself is steep and little modified (Supplementary Figure S15).

Assuming that it is possible for the crater lakes to increase in volume and approach spillover, an important stability concern are the hydrodynamic pressures on the crater walls induced by seismic shaking of the crater lake. Large-magnitude earthquakes (>M7) are common in the Aleutian Arc with one or more occurring almost annually (Sykes et al., 1981) and intense seismic shaking is a well-known mechanism for inducing failure of natural dams (Costa and Schuster, 1988). The magnitude of hydrodynamic pressure depends on the frequency-magnitude characteristics of the seismic load and the material properties of the crater walls. In addition, the seismic shaking of a lake-filled volcanic crater can induce changes in pore-fluid pressure within the crater walls and an associated reduction in shear strength. Volcanic craters that are hydrothermally altered or contain layers that transmit groundwater could be particularly susceptible to failure under seismic loading. In a study of the seismic response of earth-fill dams, Gazetas (1987) showed that ground accelerations are higher near the crest of the dam, suggesting that for lake-filled volcanic craters filled to near capacity, seismic accelerations may be sufficient to cause overtopping and water spillover.

The maximum hydrodynamic pressure (p _max) occurs at the base of the crater wall dam and can be expressed as: p m a x = 0.7 a c γ w h (1) where a c is the acceleration coefficient ( a c = a 0 g where a 0   is the maximum value of the horizontal acceleration and g is gravitational acceleration), γ w is the unit weight of water, and h is the depth of the lake at the crater wall (Pelecanos et al., 2013). This expression is valid for a dam with a vertical lake-side profile. If the internal crater wall is inclined, the hydrodynamic pressure p ( y ) is expressed as: p ( y ) = C p a c γ w h (2) where C p is a pressure coefficient (determined analytically, Chwang and Housner, 1978; Chwang, 1978), a c is the acceleration coefficient, γ w is the unit weight of water, and h is the depth of the lake (Pelecanos et al., 2013).

Earthquake induced forces on the reservoir-dam system were evaluated by Chwang and Housner (1978). Their analysis concluded that for dams with lake side facing slopes of 0–90°, the total horizontal force on the dam ( F x ) is: F x = C x ρ a 0 h 2 (3) where C x is a force coefficient (∼0.5), ρ is water density, and a 0 and h 2 are as defined above. Peak ground accelerations in the central-western Aleutian Arc associated with large megathrust earthquakes are roughly in the range of 4–8 m/s² (Wesson et al., 2007). Using these values for a 0 in the above expression and 1,000 kg/m³ for ρ , the estimated maximum force on the crater wall dam for a lake 50 m deep is 1–2 × 10⁵ N. For a lake 100 m deep, F x is 2–4 × 10⁵ N. If it is assumed that peak ground acceleration occurs in a largely bidirectional, back and forth manner, only those parts of the crater wall in the domain of motion would be subject to the maximum forces. Using a 200 m long sector of a generic crater and crater lake depths of 50 and 100 m, the above estimated forces would generate maximum crater wall pressures of only 10–40 Pa which are very low and probably not significant enough to cause failure of the crater wall by this mechanism alone.

Seismic shaking of lake-filled volcanic craters could result in liquefaction of strata within the crater walls which could be sufficient to cause crater rim failure. Several tailings dams have failed by this process (Lyu et al., 2019) and where significant portions of volcanic crater walls are composed of relatively loose, unconsolidated or hydrothermally altered material, seismically induced liquefaction could be a concern. The duration and peak ground accelerations associated with large earthquakes also can result in back-and-forth water oscillations known as seiches (McGarr and Vorhis, 1968). Seismic seiches at volcanic lakes are not widely known, but the few examples described in the literature indicate that seiches can be hazardous (Moore et al., 1966; Newhall and Dzurisin, 1988) and could be large enough to overtop crater rims which may lead to water erosion and possible dam failure.

Although it is beyond the scope of this paper to make a quantitative assessment of crater lake hydrology at each location discussed, some general commentary on the hydrologic setting of the Alaska crater lakes is warranted and provides some insight on the potential for changes in the water balance of the lakes. The primary hydrologic inputs are precipitation (snow, meltwater, rainfall; W _p ), groundwater (W _g ), and water from shallow magmatic sources (W _v ). These inputs are affected by evaporation (W _e ) and water outflow, either as direct open-channel flow (W _o ), groundwater seepage (W _gw ), or both, such that W _p + W _g + W _v = W _e + W _o + W _gw which is a general statement for the water balance of the lake (Pasternack and Varekamp, 1997; Rouwet et al., 2004). None of these variables are known with any degree of certainty for the Alaska crater lake examples discussed here. The Alaska volcanic craters are situated at the summit of their respective volcanic edifices and the crater itself defines the hydrologic catchment. Of the 16 volcanic craters examined in this study, nine are on volcanoes with significant amounts of glacier ice (>1 km²; GLIMS Consortium, 2005) and the remaining six craters are in areas that receive seasonal snow (typically September–April) that can exceed several meters depth (Brown et al., 2003; https://www.weather.gov/aprfc/Snow_Depth). The volume of glacier ice on volcanoes in Alaska has been measured only at a few locations. At Mount Spurr, about 67 km³ of glacier ice and perennial snow was present on the edifice when measured in 1981 (March et al., 1997) indicating ice volumes at similar glaciated stratovolcanoes in Alaska are significant (Waythomas, 2015). Thus, a main hydrologic input, snow and glacier ice, is present at all locations in ample amounts. An example discussed in Lahars below, shows that an increase in the thermal output at an ice and snow-clad volcanic crater (Mount Spurr) could be sufficient to cause melting and increase the water input to the crater lake. An increase in heat flux at Chiginagak volcano also led to significant melting and an increase in the volume of the crater lake eventually resulting in catastrophic lake drainage (Schaefer et al., 2008).

All volcanic crater locations receive significant precipitation and annual amounts of >1 m (water equivalent) are not uncommon (Stafford et al., 2000) although long-term weather records in the study region are available only at a few localities at sea level. If a generic volcanic crater with an area of 1 × 10⁵ m² receives 1 m of snow accumulation, the approximate snow-water equivalent (SWE) is about 5 × 10⁴ m³ of water if the SWE of 1 m of snow is about 50 cm water (Sturm et al., 2010). Thus, snowfall could represent a significant input to the Alaska crater lakes although some of this water will be lost to evaporation and groundwater seepage. These components of the water balance are difficult to estimate and although various springs have been observed (Motyka et al., 1993; Evans et al., 2015), little is known about water flux or the state of groundwater at any of the volcanic craters discussed here.

Systematic measurements of water levels in the Alaska crater lakes have not been accomplished, although opportunistic observations, photographs, and satellite data have provided a general sense of the presence and extent of lakes in some locations as described in Crater Lakes in Alaska above. Evidence of significant changes in water levels have been noted at Mount Spurr, Crater Peak, Chiginagak, Korovin, and north crater on Cerberus volcano. The summit craters at Mount Spurr and Chiginagak, and possibly Korovin and north crater have been affected by changes in the thermal output of the volcano. A lake was present at Crater Peak prior to the eruptive activity in 1992 and has not reappeared since suggesting that it too may have been a result of an increase in heat flow resulting in local snow and ice melt. Water level observations of Katmai Lake were made in the 1970s and from July 5, 1975, to August 20, 1977, the water level increased by about 6.5 m or an average rate of 3 m/yr (Motyka, 1978). Apparently, Katmai Lake drained sometime between 1919 and 1923 (Fenner, 1930). None of the other cater lakes exhibit any obvious evidence for fluctuations in lake level but it is clear from the few available observations that significant changes in water volume can occur.

The results of peak discharge estimates for water floods produced by crater wall collapse are given in Table 3. The estimates are based on maximum peak discharge associated with a fully formed trapezoidal breach calculated with Eq. 11, maximum peak discharge for rapid breach formation calculated with Eq. 6, and maximum peak discharge calculated with Eq. 7 where breach erosion rate k is set to 1, 10, and 100 m/h.

The largest peak discharge values result from application of Eq. 6 where breach formation is essentially instantaneous. These values are an upper bound on peak discharge and thus may be taken as representing the worst-case condition. The peak discharge values obtained from Eq. 6 are similar to those resulting from Eq. 11 where a trapezoidal-shaped breach is assumed throughout each crater rim failure flood event.

Values for maximum peak discharge determined from Eq. 7 are shown in Figure 4. Also shown in Figure 4 are the maximum peak discharges for other large outburst floods and lahars in Alaska and Iceland. Flood magnitudes associated with water-filled volcanic craters in Alaska are comparable to these volcanogenic flows and river floods and this highlights the potential significance of the Alaska examples.

Variable breach erosion rates in Eq. 7 yield much smaller values for maximum peak discharge compared to estimates based on nearly instantaneous breach formation (Table 3). Although it is difficult to accurately determine the erosion rate of a crater rim a priori, the discharge estimates in Table 3 indicate the strong dependence between erosion rate and peak discharge. There may be situations where it is possible to reasonably estimate the breach erosion rate and thus application of Eq. 7 in these situations would be warranted.

Expulsion of water beyond crater rims could occur during larger eruptions involving substantial amounts of volatile-rich magma, generally VEI 3 or larger, but possible for eruptions as small as VEI 1 (Mastin and Witter, 2000; Rouwet and Morrissey, 2015). Explosive displacement of water from crater lakes can take on several forms from small seiche-like or splash displacements that may or may not overtop the crater rim such as those that occurred during the initial unrest at Karymskoye Lake, Kamchatka in 1996 (Belousov and Belousova, 2009) and at Ruapehu volcano, New Zealand in 1995 (Lecointre et al., 2004; Kilgour et al., 2010) to wholesale evacuation of the entire crater lake. A numerical modeling study by Morrissey et al. (2010) indicated that if crater lake water depth is > 90 m and eruption pressures are <10 MPa, subaqueous jetting capable of breaching the water surface is unlikely to occur, whereas in situations where the eruption pressure is > 10 MPa, explosive jetting through the water column and displacement of water can occur. Given these approximate constraints, physical disruption of crater lakes in Alaska should be expected during future eruptions should they occur at the volcanoes discussed.

Complete evacuation of a crater lake occurred during the 2008 eruption of Kasatochi volcano (Figure 6) and resulted in some interesting and possibly diagnostic deposits (Figure 7). Kasatochi as described previously has a summit-defining crater that hosts a lake that today is about 8.4 × 10⁵ m² and has a volume of about 1.7 × 10⁷ m³. Although the pre-2008 lake was slightly smaller, the volume was likely on the order of 10⁷ m³ (Figure 6). This volume of water was explosively discharged from the crater during the early phase of the 2008 Kasatochi eruption (Waythomas et al., 2010a; Scott et al., 2010). Although not widespread on Kasatochi Island, the first of the 2008 eruptive deposits to accumulate were massive grey muddy tephra deposits and multiple beds of pumice rich coarse ash and mud that collectively are up to 50 cm thick. When freshly exposed by shovel, these deposits oxidize rapidly and take on a yellow-brown-gray color indicating that the muddy components were probably chemically reduced when deposited (Scott et al., 2010). The pH of these deposits was 3–4.5, lower than all other 2008 eruptive deposits (Wang et al., 2010). A plausible explanation for these deposits is that they represent erupted lacustrine sediment from the floor of the crater lake. The muddy fall deposits are overlain by massive pyroclastic-flow deposits at least 10 m thick, that contain juvenile pyroclasts of light grey andesite and lithic debris (Figure 7). Various types of unburned organic matter (twigs, stems, moss, and grass) are dispersed throughout the lower 1–2 m of the pyroclastic-flow unit in several locations (Figure 7). This material represents the vegetated surface on the flanks of the volcano that was overrun and eroded by pyroclastic flows. Wave erosion of the Kasatochi coastline exhumed areas where wooden observation structures had been constructed by biologists working on the island; lumber remains in these areas is also unburned or charred. The presence of unburned organic matter and wood in the basal part of the pyroclastic-flow sequence indicates that these deposits were at low temperature (<300 °C) when emplaced. It is reasonable to conclude that water, water vapor, or both were components of the initial erupting mass and present in significant quantities to cool the eruptive mixture including pyroclastic flows emplaced during the early part of the eruption. As the eruption began to subside and water began reentering the crater, ejection of ballistic particles was noted (Waythomas et al., 2010a). This may have been a result of a greater degree of magma-water interaction as the supply of water increased.

Eruptive removal of the crater lake within the south crater of Korovin volcano was suspected in 2002 and 2004 (McGimsey et al., 2008). The south crater typically hosts a small crater lake with an area of about 1.6 × 10⁴ m² (Table 1; Supplementary Figure S13). Images of south crater in 2002 and 2004 show grey, apparently fine-grained deposits on the upper east flank of the cone and these deposits exhibit flowage features (digitate lobes, levees, and shallow channels; Figure 8) indicative of water-saturated flow. Brief eruptive bursts at Korovin’s south crater are somewhat common historically and are likely phreatic or phreatomagmatic in origin (McGimsey et al., 2008). During these bursts, it is inferred that wet mixtures of ash and debris are emplaced on the upper part of the cone just beyond the crater rim. Although yet unconfirmed with before and after observations, the crater lake is also likely partially or entirely removed during these types of events.

Discharge of water from crater lakes that led to the development of lahars has only been documented a few times in Alaska. The most noteworthy example is at Chiginagak volcano where acidic water drained via an ice dam and produced a lahar and water flood downstream from the crater (Figure 9; Schaefer et al., 2008; Schaefer et al., 2013). The volume of water released was about 3.8 × 10⁶ m³ (Schaefer et al., 2008). Because the fluid component of the flow was acidic (pH at least as low as 2.9) and it liberated acidic aerosols along its flow path, considerable impacts to the riparian ecosystem and to Mother Goose Lake occurred (Schaefer et al., 2008; Schaefer et al., 2013). Apparently, release of acidic water from Chiginagak crater is a recurring event as lake sediment cores from Mother Goose Lake reveal multiple layers of orange-colored sediment interpreted as acidic flood deposits (Kassel, 2009).

Watery debris flows were observed on the upper part of the Mount Spurr cone in July 2004 (Figure 10). The flows coincided with a period of elevated heat flow at the summit of Mount Spurr that melted a substantial amount of ice and snow at the summit to reveal a summit crater (Coombs et al., 2006). As described in an earlier section of this paper, a small lake developed in the crater. A hydrologic connection between the crater lake and the debris flows observed in July 2004 has not been determined. A plausible explanation for the flows is that they formed by outflow of ponded subglacial water stored on the Mount Spurr edifice that was produced by the elevated heat flow and if so, do not record drainage from the crater lake. Coombs et al. (2006) proposed forceable ejection of water from the crater lake resulting from rapid ice collapse into the water body. For this to occur, a subglacial channel network able to convey the debris-laden water would have to develop either prior to or coincident with ice collapse. Such networks are inferred at the base of thick glaciers (>100 m) but in these situations water release is related to the glacier reaching a flotation condition because of subglacial ponding of water to roughly 0.9 times the ice thickness (Björnsson, 1974; Nye, 1976). The prolonged period of elevated heat flow at the summit of Mount Spurr could have changed the permeability conditions of the snow and ice cover on the edifice such that forceable ejection of water produced the debris flows; however, the hydrologic connection between the crater lake and the flows remains unclear.

Explosive eruptions through water commonly produce ring-shaped, radially spreading, turbulent clouds known as base surges (Moore, 1967; Waters and Fisher, 1971; Rouwet and Morrissey, 2015). Base surges move along the surface at speeds on the order of tens of meters per second and can flow as much as 10 km from the eruption site (Mastin and Witter, 2000). Although base surges were observed during the 2016–2017 eruption of Bogoslof volcano (Waythomas et al., 2020) and during the 1977 eruption at Ukinrek Maars (Self et al., 1980), they have not been documented during historical eruptive activity at any of the crater lakes discussed here. It is possible that base surges developed during the 2008 Kasatochi eruption as surge deposits are a component of the sequence of eruptive deposits (Waythomas et al., 2010a; Scott et al., 2010).