# Definition, formation and rupture mechanisms of water pockets in alpine glaciers: Insights from an updated inventory for the Swiss Alps

Christophe Ogier, Mauro Fischer, Mauro A Werder, Matthias Huss, Mauro Hupfer, Mylène Jacquemart, Olivier Gagliardini, Adrien Gilbert, Leo Hösli, Emmanuel Thibert, et al.

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## To cite this version:

Christophe Ogier, Mauro Fischer, Mauro A Werder, Matthias Huss, Mauro Hupfer, et al. Definition, formation and rupture mechanisms of water pockets in alpine glaciers: Insights from an updated inventory for the Swiss Alps. Journal of Glaciology, 2025, 71, pp.e82. 10.1017/jog.2025.43. hal-05223624

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Distributed under a Creative Commons Attribution 4.0 International License
Definition, formation and rupture mechanisms of water
pockets in alpine glaciers: insights from an updated
inventory for the Swiss Alps

Christophe OGIER¹,², Mauro FISCHER³,⁴, Mauro A. WERDER¹,², Matthias HUSS¹,²,⁵, Mauro HUPFER³, Mylène JACQUEMART¹,², Olivier GAGLIARDINI⁶, Adrien GILBERT⁶, Leo HÖSLI¹,², Emmanuel THIBERT⁶, Christian VINCENT⁶, Daniel FARINOTTI¹,²

¹Laboratory of Hydraulics, Hydrology and Glaciology (VAW), ETH Zurich, Zurich, Switzerland

²Swiss Federal Institute for Forest, Snow and Landscape Research (WSL), bâtiment ALPOLE, Sion,
Switzerland

³Institute of Geography, University of Bern, Bern, Switzerland

⁴Oeschger Centre for Climate Change Research, University of Bern, Bern, Switzerland

⁵Department of Geosciences, University of Fribourg, Fribourg, Switzerland

⁶Univ. Grenoble Alpes, CNRS, INRAE, IRD, Grenoble INP, IGE, 38000 Grenoble, France

Correspondence: Christophe Ogier <ogier@vaw.baug.ethz.ch>

ABSTRACT.

The term "water pocket" describes invisible en- and subglacial water reser-
voirs that can cause sudden glacial outburst floods. However, there is currently
no consensus on its definition and the formation and rupture mechanisms
of water pockets remain poorly understood. This study aims to understand
the mechanisms behind water pocket outburst floods (WPOFs) from alpine
glaciers by analyzing their spatial and temporal distribution, pre-event mete-
orological conditions, and the glacio-geomorphic features of the glaciers from
which the floods originate. To this end, we updated an inventory of known
WPOFs in the Swiss Alps to 91 events from 37 individual glaciers. Most
WPOFs occurred between June and September, likely linked to meltwater in-
put. Meteorological data indicate anomalously high temperatures during the
days preceding most events and heavy precipitation on 25% of days for which

<footnote>This is an Open Access article, distributed under the terms of the Creative Commons Attribution licence
(http://creativecommons.org/licenses/by/4.0), which permits unrestricted re-use, distribution and reproduction, provided
the original article is properly cited.</footnote>
WPOFs occur, indicating that water pockets typically rupture during periods of high water input. We propose four mechanisms of water pocket formation: temporary subglacial channel blockage (which is the mechanism suggested most often for our inventory), hydraulic barriers, water-filled crevasses, and accumulation of liquid water behind barriers of cold ice (thermal barriers). Overall, our analysis highlights the challenge of understanding WPOFs due to the sub-surface nature of water pockets, emphasizing the need for field-based research to improve their detection and monitoring.

# 1 INTRODUCTION

The term "water pocket" is often used as an umbrella term to describe the unknown origin of glacial outburst floods. To this day, there is no consensus on the definition of water pockets in the literature. The term "water pocket" can refer to englacial and subglacial water bodies of different sizes and shapes. For instance, some studies use the term water pocket to refer to centimeter-scale water inclusions in glaciers (?????), whereas others use it for a water reservoir with a substantial volume (????). Here, we define a glacial water pocket as an *englacial or subglacial water-filled cavity with a volume larger than 1000 m³*. In that sense, we align with the wording used in previous studies (????), where the term water pocket was used to describe a volume of water large enough to potentially trigger a flood. ? introduced the term water pocket outburst floods (WPOFs) for glacial outburst floods originating from the rupture of a water pocket. We will keep the same term and abbreviation in this article.

WPOFs are different from glacier lake outburst floods (GLOFs, ?????), for which the water giving rise to a flood stems from a detectable reservoir located either in the glacier forefield (i.e. proglacial lake, see ?, for a review), at the surface of the glacier (i.e. supraglacial lake, e.g. ???), at the glacier margin (i.e. ice-marginal lake, e.g. ?), or at the glacier base (i.e. subglacial lake, see ?). Note that GLOFs from ice-dammed lakes are often called jökulhlaups, an Icelandic term literally translating to "glacier run". We exclude subglacial lakes where geothermal heat flux is a significant contributor to lake water from our definition of water pockets since this process is largely absent in mountain glaciers and has already been extensively studied in the literature (e.g. ??). WPOFs are also in contrast to so-called spring events, which are glacial floods triggered by the sudden destabilization and re-organization of the englacial and
subglacial drainage system (e.g. ??). Spring events occur when the state of the drainage network in spring shifts from an inefficient to an efficient drainage system, due to a sudden input of water from melt or/and precipitation (e.g. ?). Note that although spring events and WPOFs differ in the nature of their water reservoirs, observational evidence from hydrographs of pro-glacial streams makes it challenging to distinguish between the two in practice.

Proglacial, supraglacial, and ice-marginal lakes are generally well-visible to observers. As a consequence, the physical processes of the filling and the rupture of these water reservoirs are relatively well understood. GLOFs from proglacial lakes dammed by moraines can be triggered by the collapse of the moraine due to hydrostatic pressure and water infiltration, or by rapid mass movements into the lake, which produce displacement waves that overtop the dam, and eventually lead to the breaching of the dam (?). GLOFs from ice-dammed lakes occur when the ice dam weakens and breaks due to flotation by hydrostatic pressure (e.g. ?), when the dam is incised from below and leads to enlargement of subglacial outflow channels (?), or when the dam is overtopped by either a progressive increase in the lake level (e.g. ??), or when the lake surface is impacted by avalanches and landslides (e.g. ??). Unlike GLOFs, WPOFs are more difficult to study due to the hidden nature of water pockets.

The best documented water pockets are those in Glacier de Tête Rousse in France. In 1892, a WPOF from this glacier claimed the lives of 175 people. The water pocket is thought to have formed as a supraglacial lake during a period of negative mass balance between 1867 and 1878. In a subsequent period of positive mass balance until 1892, the ice- and snow-covered lake may have become englacial (?). This resulting water pocket suddenly drained after a mechanical rupture of the glacier front. In 2010, a water pocket was again discovered at Tête Rousse between the glacier base and the bedrock (? ??), and was artificially drained to limit the risk of downstream flooding (?). The lake likely formed as a subglacial cavity that filled with meltwater and grew due to outward ice-creep induced by water pressure larger than overburden ice pressure (?). The water was trapped at the transition between cold and temperate ice, a mechanism that likely also played a role in the 1892 event (?). To our knowledge, the water pocket discovered at Tête Rousse in 2010 is the only one worldwide that was investigated using geophysical methods (??), highlighting the challenges in studying these phenomena in the field and clarifying why there is limited understanding of outburst floods from water pockets to date.

The frequency and characteristics of (all types of) glacier-related floods in the Swiss Alps was investi-
gated by ?. The study found that 60-70% of the ca. 100 outburst floods documented at that time were
related to glacier- and moraine-dammed lakes and that 30-40% were likely related to the rupture of water pockets. Although the glacier thermal regime was not discussed in the study, most of the glaciers that had WPOFs were assumed to be temperate. This supports the existence of subglacial and englacial cavities in temperate ice, whereas the Tête Rousse case exemplifies a water pocket formed in polythermal ice, with cold ice acting as a barrier to liquid water. Most of the glacier floods documented in ? occurred during summer, suggesting no fundamental difference between the glacial drainage network associated with lake and water pocket outbursts. However, in most cases the en-/subglacial water reservoir was not directly observed. The author states: "*The lack of clarity in the term water pocket illustrates perfectly the dilemma of the glaciologist who is faced with a situation which he [she/they] does not fully understand*".

In this study, we seek to improve our understanding of the mechanisms leading to WPOFs from moun-
tain glaciers by analyzing the spatial and temporal distribution as well as the glacial, geomorphological, and
pre-event meteorological characteristics of water pocket ruptures identified in the Swiss Alps. We review
the literature on glacial water pockets and extend the glacier floods inventory analysed by ? for WPOFs
up to 2023. Based on available event documentation and a literature review, we attempt to understand
the distinct mechanisms that may lead to water pocket formation and rupture.
<figure><img src="image_3.png" /><figcaption>**Fig. 1.** Examples of water pocket outburst floods in the Alps documented during the last decades. $V_{\text{flood}}$ refers to the estimated WPOF volume. The photographs of the cavity at Hubelgletscher, Minstigergletscher and Tête Rousse were taken after the outbursts. At Oberer Grindelwaldgletscher, Feegletscher and Griesgletscher, the photographs were taken during the outburst floods. With the exception of Glacier de Tête Rousse (polythermal), all glaciers were assumed to be temperate.</figcaption></figure>
# 2 DATA AND METHODS

## 2.1 Water pocket outburst floods inventory

As a basis for this study, we used the inventory of hazardous glaciers in Switzerland by ? that spans 304 years from 1699 to 2003 and includes the WPOF inventory from ?. We extended and updated this inventory for WPOFs in Switzerland from three main sources: (i) the updates of glacier floods reported by ?, (ii) the meta-analysis conducted by ?, and (iii) various cantonal reports compiled by ?. The updated WPOFs inventory contains a total of 91 recorded events originating from 37 different glaciers. This more than triples the number of WPOFs reported by ? (n = 26).

The updated WPOF inventory includes the following information: the glacier name where the outburst originated, the name of the river that experienced a flood, the date of the WPOF event, the suggested type of outburst mechanism described in Section ??, the water pocket volume, the flood volume, the peak discharge, the damages reported, and the sources (see the code and data availability statement to access the inventory). In most cases, not all of the information is available. For the 25 events with unknown exact dates, a date range is provided to account for uncertainties. Note that in most of the documentation found for the reported events, the wording "water pocket rupture" is used to describe glacier-related floods of unclear origin, i.e. glacier floods that occur in the absence of previously recognized proglacial, supraglacial or ice-marginal lakes. Among all the recorded events, 64 events have direct observations of the flood at the glacier tongue, while 27 events are characterized as speculative because of the lack of direct observations. Two WPOFs caused the death of three people in total (Rhonegletscher in 1934 and Vadret da l'Alp Ota in 2006), and infrastructure damage was reported for 43 events. Photographic evidence of water pocket reservoirs is available for only two events: Hubelgletscher in 2004 and Minstigergletscher in 2008 (both are shown in Fig. ??), where ice-surface collapse exposed the cavity containing the liquid water before the WPOF. Additionally, a sketch exists for the water pocket of Glacier du Mont-Miné in 1943 (?), and a description of the cavity is available for Mellichgletscher in 1920 (?).

We consider our inventory of 91 WPOFs in the Swiss Alps to be comprehensive for documented cases, but we do not expect it to be complete. Rather, we assume that a large number of WPOFs – especially smaller ones – occurred unnoticed. We expect the inventory to be biased in two main aspects. First, we expect a spatial bias, with WPOFs from glaciers close to hydropower or tourism infrastructure, as well as settlements being more likely to be reported. Second, we expect a temporal bias, with more recent WPOFs
having a greater chance of being documented (see also ?, for biases in reporting GLOFs worldwide).
Throughout the 20th century, we assume that observational biases decreased due to the expansion of
environmental monitoring in Switzerland (e.g. from the Swiss Federal Office for the Environment) and the
growing interest in glacier-related natural hazards (e.g. ???). Figure ??b shows the temporal evolution in
the five-year average of the publication time lag, i.e. the time interval between a WPOF occurrence and
its first report in the literature. This average decreases over time to less than one year from 2015 onward.
We interpret this trend as a proxy for research and reporting activity, with shorter publication time lags
indicating greater interest in WPOFs reporting. Note also that the inability to rule out spring events based
solely on pro-glacial observations represents a potential source of misclassification in the WPOFs inventory.

## 2.2 Meteorological analyses

We used daily gridded air temperature and precipitation data from MeteoSwiss from 1961 to 2022 (?) to analyze the meteorologic conditions prior to the 32 WPOFs for which precise event dates are known within this period. We used the *TabsD* (daily mean temperature) and *RhiresD* (cumulative precipitation) products with a spatial resolution of 1 km. Temperature and precipitation data are extracted from the grid cell containing the centroid coordinates of the Swiss glacier inventory (SGI) 2016 (?). For each WPOF we then calculated the mean air temperature anomaly and the cumulative precipitation anomaly for a period between 0 (day of the outburst) to 21 days prior to the outburst (Fig. ??a and b). Air temperature and precipitation anomalies are calculated relative to their mean for the same period (021 days prior to the event). These mean values correspond to the 30-year climate reference periods (19611990 or 19912020), depending on the year of WPOF occurrence. In addition, we classified the cumulative precipitation for the days of the WPOFs into the six categories defined by MeteoSwiss (Fig. ??c): no rain (0 mm), weak (0.1 to 2 mm), moderate (2.1 to 10 mm), strong (10.1 to 30 mm), very strong (30.1 to 50 mm) and heavy rain (> 50 mm).

## 2.3 Glacio-geomorphic analyses

To test whether specific factors might control the occurrence of WPOFs, we also analysed a set of glacier-wide glacio-geomorphic variables inferred for the glaciers at the time of the events. These variables were selected for their potential influence on meltwater production and glacier hydraulics without prior assumptions. We then compared these variables with their values for all Swiss glaciers. This investigation was
already attempted by ? who documented the glacier surface area, mean glacier slope, and mean annual air temperature at the equilibrium line for 26 glaciers with reported WPOFs. The latter study did not identify any geomorphological characteristics that explain WPOFs occurrence. The variables analysed in our study include the relative debris cover, the accumulation area ratio, the glacier area, and the mean glacier elevation, aspect and slope. For glaciers with no reported WPOFs, the variables are taken from the SGI2016. For glaciers with reported WPOFs, all variables are taken from the Swiss glacier inventory that is temporally closest to the respective WPOF event (SGI1850, SGI1973 and SGI2016; ???)). For relative debris cover and slope prior to the SGI2016, we used the values reconstructed by ?. Additionally, we analyzed the mean ice surface velocities taken from ? that correspond to the average velocities between 2017 and 2018 for both categories (WPOF and non-WPOF glaciers). We do not consider historic data for ice surface velocities prior to 2017 as no corresponding data set is available.

# 3 OCCURRENCE AND FREQUENCY OF WATER POCKET OUTBURST FLOODS IN SWITZERLAND

## 3.1 Spatio-temporal distribution and magnitude of reported WPOFs

The spatial distribution of documented WPOF events in Switzerland is shown in Fig. ??a. As already shown by ?, the vast majority of inventoried WPOFs were observed at glaciers in the Bernese and especially Pennine Alps. Twenty glaciers (i.e. 54% of glaciers with known WPOFs) show more than one reported WPOF event.

The temporal distribution of WPOFs reveals that there is a clear seasonal trend. Almost all WPOFs (68 out of 75 events with reported dates) occurred between June and September (Fig. ??c). This was already reported by ? and suggests that water input from snowmelt, ice melt or rainfall is important for the filling and the rupture of water pockets. However, we cannot conclude about interannual trends in WPOFs occurrence because we could not quantify the suspected temporal observational biases in reporting WPOFs.

Information on flood volume is available for 20 WPOFs (Fig. ??b). These volumes are either derived from measurements at gauging stations located a few kilometers downstream, or estimated by the available reports. The volume estimation method is not known for most cases because the original source or reference is no longer accessible (e.g. ?). In many cases, these volumes come with high uncertainties that we did not quantify because in general the flood duration and the stream baseflow are not precisely known, although
both are needed to derive the flood volume. The average flood volume for the 20 events is 3.1×10⁵ m³, with a minimum of 2250 m³ and a maximum of 1.6×10⁶ m³ (Glacier du Mont Miné in 1943 (?)). The average WPOF volume reported in our dataset is ten times smaller than the average GLOF volume (2.97×10⁶ m³, WPOFs excluded) reported by ?, who analysed a set of 89 GLOFs that occurred in the Swiss Alps between 1560 and 2019. Due to the lack of measurements on WPOF volumes, this average WPOF volume should be only considered as a rough representation for the whole water pocket population.

## 3.2 Climatic and meteorological characteristics of reported WPOF events

Water pockets are filled by water from rain, ice melt, and snowmelt; their formation is therefore influenced by weather and potentially by climate. However, due to unquantified observational biases in our inventory, it is not possible to infer any potential climatic influence. Nevertheless, there are instances where specific climatic conditions have impacted water pocket formation, such as the case of the water pocket discovered at Glacier de Tête Rousse (France) in 2010. The formation of this water pocket was caused by the glacier's polythermal temperature regime (?), which was explained by the combination of particular climate and glacio-geomorphic conditions (see Section ?? for more details).

The intra-annual timing of WPOFs shows some correlation with meteorological factors. The analysis of the meteorological conditions of 32 WPOFs occurring between 1961 and 2022 shows a positive mean temperature anomaly (+1°C) during the three weeks prior to the WPOFs, with a maximum average temperature anomaly of +2°C reached during the day prior to the outburst (Figure ??a). This suggests that above-average meltwater input caused the critical water volume, i.e. the water volume threshold above which the water pocket ruptures, to be reached in a few days before most WPOFs documented in our inventory. However, this analysis does not provide information to estimate the filling duration prior to water pocket rupture, since the development of the cavity before the rupture is unknown. Conversely to the air temperature signal, there is no anomaly in the cumulative precipitation during the three weeks prior to the WPOFs (Fig. ??b). On the days of the WPOF events (n=32), there was no or only weak precipitation (<2 mm) in 20 cases, moderate precipitation (2.1 to 10 mm) in 4 cases, and strong to heavy precipitation (>10mm) for 8 cases (Fig. ??c). Here, precipitation refers to either rain or snowfall, and occurred as snow for 6 WPOFs.
<figure><img src="image_4.png" /><figcaption>**Fig. 2.** Insights from our extended and updated WPOF inventory for the Swiss Alps. a) Spatial distribution of documented events. The inventory contains a total of 91 events from 37 glaciers, with 20 glaciers (yellow stars) experiencing repeated events. b) Frequency (bars) and volume (blue dots) of documented WPOFs. The frequency is given as the number of WPOFs per five-year interval. The publication time lag is averaged per five-year interval and refers to the duration between the occurrence of an event and the date it is first reported in the literature. The WPOF magnitude is shown for all 20 events with available information on flood volume. c) Seasonal distribution of reported WPOFs.</figcaption></figure>
<figure><img src="image_5.png" /><figcaption>C Cumulative precipitation on WPOF event day</figcaption></figure>

<figure><img src="image_6.png" /><figcaption>b Daily cumulative precipitation anomaly prior to WPOFs</figcaption></figure>

<figure><img src="image_7.png" /></figure>

**Fig. 3.** Meteorological conditions prior to and during documented WPOFs in the Swiss Alps since 1961 with exactly known event dates (n = 32). a) Daily mean air temperature anomaly and b) cumulative precipitation anomaly for 0 to 21 days prior to WPOFs. For both panels, the mean (blue dot), the median (red dash), the 75th and the 25th percentile are shown (i.e. the interquartile range). The red crosses represent outliers that lie beyond the lower and upper adjacent values, which are the smallest and largest data points within 1.5 times the interquartile range from the bottom or top of the box. c) Cumulative precipitation over WPOF glaciers on the event day, classified into six categories (from no precipitation to heavy precipitation) according to MeteoSwiss thresholds.
## 3.3 Glacio-geomorphic characteristics of WPOF glaciers

Figure ?? shows the accumulation area ratio, relative coverage with supraglacial debris, the glacier's average slope, mean aspect, surface area, median elevation, and mean ice surface velocity, for both Swiss glaciers with at least one reported WPOF and for all glaciers in Switzerland. The results show that (i) the median area of glaciers with reported WPOF events is thirty times larger than the median area of all Swiss glaciers (2.72 km² vs. 0.09 km²), (ii) the average ice surface velocity is 2.5 times higher for glaciers with WPOFs than it is for the other glaciers (12.88 vs. 4.98 m yr⁻¹), and (iii) the median of the average slope is 11ř less steep for glaciers with reported WPOFs compared to all other glaciers (28.2ř). We conducted Mann-Whitney U tests to assess the significance of the differences between the statistical distribution of glacio-geomorphic variables of glaciers with and without reported WPOFs. Our findings revealed that all variables, except aspect, exhibited statistically significant differences at the 0.05 level. However, we question the interpretability of this significance for two reasons. First, the observations could potentially be influenced by observational biases, as larger WPOFs from larger glaciers might be more often recognized and reported than those from smaller glaciers. Second, the number of glaciers with known WPOFs (n=37) may be too small compared to the entire glacier population in Switzerland (n=1400) to draw robust conclusions. We conclude that, based on our dataset, there is no obvious set of glacier-wide glacio-geomorphic variables that would allow distinguishing between glaciers that are prone to WPOFs and glaciers for which WPOFs are unlikely to occur. This result aligns with ?, and we suggest that the occurrence of WPOFs is likely controlled by smaller-scale topographic or glacio-geomorphic features, for which there is too little information in the analysed variables evaluated at the glacier-wide scale. Most importantly, there is no information on the exact location of the water pockets for most cases, which prevents a smaller-scale analysis.
<figure><img src="image_8.png" /><figcaption>**Fig. 4.** Distribution of glacier-wide glacio-geomorphic variables for Swiss glaciers with reported WPOFs (n = 37, in black) based on the temporally closest Swiss glacier inventory (???) as well as data from ? and ?, and for all glaciers in Switzerland (in blue) based on the latest Swiss glacier inventory (SGI2016, ?). Variable values are normalized. The median (circle with dot) and the interquartile range (box) are shown for each variable. The unfilled circles represent outliers that lie beyond the lower and upper adjacent values, which are the smallest and largest data points within 1.5 times the interquartile range from the bottom or top of the box. The area is log-transformed before normalization to improve readability.</figcaption></figure>
# 4 HYPOTHESES FOR THE FORMATION OF GLACIAL WATER POCKETS AND THEIR OUTBURSTS MECHANISMS

We propose four mechanisms describing the formation of water pockets in alpine glaciers, based on event descriptions from 17 documented WPOFs (see Table ??) in our inventory and a literature review. These formation mechanisms comprise all the processes involved in the evolution of the water pocket, from its growth and filling to its eventual rupture. The documented formation mechanisms can be divided into the following three categories: 1) temporary blockage of subglacial channels (11 WPOFs), 2) hydraulic barrier (5 WPOFs), and 3) water-filled crevasses (1 WPOF). The mechanism of formation for the remaining 75 events remains unknown. In addition, we present a fourth formation mechanism that hasn't been described for any WPOF in the Swiss inventory but that is documented from the case study of Glacier de Tête Rousse in the French Alps (?): 4) thermal barrier. Figure ?? schematically shows the four water pocket formation mechanisms we propose.

## 4.1 Temporary blockage of subglacial channels

Temporary blockage of subglacial channels by ice blocks collapsing from the channel roof has the potential to accumulate water within the glacier, which can be released suddenly when the temporary ice dam breaks (???). Typical summer discharges of proglacial melt water streams for medium-sized alpine glaciers can easily be in the order of 10 m³ s⁻¹ (??). Thus, it would only take a few hours to accumulate a water reservoir of a few thousands of m³ in a blocked subglacial channel, assuming the presence of a pre-existing cavity. This mechanism is illustrated in Figure ??a. Note that for the roof collapse to occur, the subglacial channel needs to have temporarily free-surface flow.

According to event descriptions and information stored in our inventory (see Table ??), temporary blockage of subglacial channels is likely to have occurred in 11 cases, namely at Rhonegletscher in 1900, 1934 and 1947, at Glacier du Mont-Miné in 1952, at Bas d'Arolla in 1974, at Findelgletscher in 1943 and 2017, at Hubelgletscher in 2004, at Oberer Grindelwaldgletscher in 2011 (twice), and at Griesgletscher in 2023. Information in terms of event size is only available for three cases: at Findelgletscher in 2017, Oberer Grindelwaldgletscher in 2011 (?), and Hubelgletscher in 2004 (?), the estimated total flood volumes excluding baseflow were estimated at 17'000 m³, 300'000 m³, 21'500 m³ respectively. The corresponding peak discharges were of 15 m³ s⁻¹ (hourly mean value at 1.5 km downstream of the glacier portal, this is four times larger than the baseflow; Fig. ??b), 73 m³ s⁻¹ (4 min resolution, 4 km downstream, five times
<table><caption>**Table 1.** List of 17 water pocket outburst floods (WPOFs) that can be assigned to one of the mechanisms proposed in Section ???. The remaining 74 events of the WPOFs inventory lack sufficient evidence to be assigned into any of the proposed mechanisms. The abbreviations "temp. bloc", "hydraulic bar." and "w.-f. crevasse" refer to temporary blockage of subglacial channels, hydraulic barriers and water-filled crevasses, respectively. The thermal barrier mechanism is absent from the Swiss-wide WPOFs inventory. Qualitative observations are marked with a "*", and quantitative observations (i.e. from measurements) are marked with a "+". "obs" refers to visual observation. All glaciers are assumed to be temperate. References for the individual events are provided in the full inventory (see Section Code and data availability).</caption><thead><tr><th>Glacier</th><th>WPOF date<br>(yyyy-mm-dd)</th><th>Mechanism<br>(see Section ??)</th><th>Hydrograph</th><th>Circular<br>crevasses</th><th>Ice<br>blocks</th><th>Cavity<br>obs.</th></tr></thead><tbody><tr><td>Rhonegletscher</td><td>1900-12-??</td><td>temp. bloc.</td><td>-</td><td>-</td><td>-</td><td>no</td></tr><tr><td>Rhonegletscher</td><td>1934-07-17</td><td>temp. bloc.</td><td>-</td><td>-</td><td>yes*</td><td>no</td></tr><tr><td>Findelgletscher</td><td>1943-07-23</td><td>temp. bloc.</td><td>-</td><td>-</td><td>yes*</td><td>no</td></tr><tr><td>Findelgletscher</td><td>2017-07-04</td><td>temp. bloc.</td><td>cut-off<sup>+</sup>,*<br>see Fig. ??b</td><td>before<sup>+</sup><br>after<sup>+</sup></td><td>-</td><td>no</td></tr><tr><td>Rhonegletscher</td><td>1947-08-09</td><td>temp. bloc.</td><td>cut-off*</td><td>-</td><td>yes*</td><td>no</td></tr><tr><td>Glacier du Mont-Miné</td><td>1952-08-04</td><td>temp. bloc.</td><td>-</td><td>-</td><td>yes*</td><td>no</td></tr><tr><td>Bas d'Arolla</td><td>1974-06-10</td><td>temp. bloc.</td><td>sharp rise<sup>+</sup></td><td>-</td><td>-</td><td>no</td></tr><tr><td>Hubelgletscher</td><td>2004-08-04</td><td>temp. bloc.</td><td>sharp rise<sup>+</sup><br>see Fig. ??d</td><td>after*</td><td>-</td><td>photograph</td></tr><tr><td>Oberer<br>Grindelwaldgletscher</td><td>2011-06-29</td><td>temp. bloc.</td><td>sharp rise<sup>+</sup></td><td>-</td><td>-</td><td>no</td></tr><tr><td>Oberer<br>Grindelwaldgletscher</td><td>2011-08-25</td><td>temp. bloc.</td><td>cut-off<sup>+</sup><br>see Fig. ??c</td><td>-</td><td>-</td><td>no</td></tr><tr><td>Griesgletscher</td><td>2023-07-31</td><td>temp. bloc.</td><td>sharp rise*</td><td>-</td><td>yes*</td><td>no</td></tr><tr><td>Glacier d'Orny</td><td>1920-06-24</td><td>hydraulic bar.</td><td>-</td><td>-</td><td>-</td><td>no</td></tr><tr><td>Glacier d'Orny</td><td>1920-06-30</td><td>hydraulic bar.</td><td>-</td><td>-</td><td>-</td><td>no</td></tr><tr><td>Glacier d'Orny</td><td>1920-07-04</td><td>hydraulic bar.</td><td>-</td><td>-</td><td>-</td><td>no</td></tr><tr><td>Glacier du Mont-Miné</td><td>1943-08-23</td><td>hydraulic bar.</td><td>modelled</td><td>-</td><td>-</td><td>sketch</td></tr><tr><td>Minstigergletscher</td><td>2008-08-21</td><td>hydraulic bar.</td><td>-</td><td>-</td><td>-</td><td>photograph</td></tr><tr><td>Mellichgletscher</td><td>1920-??-??</td><td>w.-f. crevasses</td><td>-</td><td>-</td><td>-</td><td>description</td></tr></tbody></table>
<figure><img src="image_9.png" /></figure>

**Fig. 5.** Schematic representation of the proposed four main mechanisms of water pocket formation in alpine glaciers: 1) temporary blockage of subglacial channels, 2) hydraulic barrier, 3) water-filled crevasses, and 4) thermal barrier. CTS indicates the cold-temperate transition surface, i.e. the surface along which the ice of a polythermal glacier transitions from cold to temperate.

larger than the baseflow; Fig. ??c), and 22 m³ s⁻¹ (10 min resolution, 15 km downstream, 147% of the baseflow; Fig. ??d), respectively. The empty englacial cavity at Hubelgletscher following the 2004 event was estimated to have a volume of 2100 m³ (?).

We hypothesise that the ice blocks that are necessary for temporarily forming an ice dam are most likely to stem from partial collapses of subglacial cavities rather than developing from regular subglacial channels (i.e. R-channels described in ?). Here, we use the term "subglacial cavity" to describe voids that are significantly larger than the linked cavities that are part of an inefficient subglacial drainage system (e.g. Fig. 14 in ?). We argue that a partial collapse of the roof of a subglacial cavity is likely to occur before the cavity fills with water because its geometry is known to be driven by the failure of ice lamellas at the cavity's roof (???). Collapsing ice lamellas are necessary to suddenly and temporarily block the water flow in subglacial cavities. Such collapses are unlikely to happen in subglacial channels, where channel geometry is primarily controlled by the balance between ice melt at the water-ice interface and ice creep (?).

The fact that the temporary blockage of subglacial channels is a plausible water pocket formation mechanism is also supported by both the presence of ice blocks in the proglacial stream during an outburst, and by a cut-off of the stream discharge before an outburst (e.g. ?). Floating ice blocks with signs of recent
collapse were observed in the proglacial stream of Findelgletscher in 2017 (?), and at Griesgletscher in 2023 (Fig. ??). Discharge cut-off before a WPOF was observed at Rhonegletscher in 1934 (?) and 1947 (?), at Oberer Grindelwaldgletscher in 2011, and at Findelgletscher in 2017 (Fig. ??b and c). The sharp increase in discharge at Findelgletscher (Fig. ??b) is in line with a sudden breaching of the ice dam (?). For Hubelgletscher in 2004, the hydrograph of the WPOF exhibited a sharp peak, likely due to an abrupt ice dam rupture (?). However, the distant gauging station fails to capture a discharge cut-off signal (Fig. ??d), possibly because of the high meltwater contributions from other glaciers. For all examples in Figure ??, the discharge cut-off duration indicates that the temporary blocking of the subglacial drainage was relatively short, i.e. between a few minutes (Fig. ??c) to a few hours (Fig. ??b).

In thin ice, the partial collapse of a cavity’s roof can be observed at the glacier surface through the presence of circular crevasses centred around the cavity (e.g. ????). The collapse of ice blocks is frequent during the development of subglacial cavities above subglacial channels (??). The development of circular crevasses soon after a WPOF (i.e. stage 4 in Fig. ??a) was observed at Hubelgletscher in 2004 (Fig. ??) and at Findelgletscher in 2017.

In the European Alps, the formation of circular crevasses – potentially indicative of water pocket rupture – is increasing with rising air temperature, as the latter accelerates glacier thinning and warm air advection upstream of glacier portals (???). So far, however, it remains unclear whether WPOFs caused by temporary blockage of subglacial channels are occurring more often because of atmospheric warming. This is because no clear trend was observed in the frequency of documented WPOFs in past decades (Fig. ??b), and because the specific mechanisms behind the initiation of circular crevasses are not yet fully understood.

## 4.2 Hydraulic barrier

Water accumulates at the glacier bed in areas with a local minimum in the hydraulic potential. The ice surrounding these areas of low hydraulic potential can act as hydraulic barriers for the subglacial water flow, and we hypothesise that these hydraulic barriers constrain potential locations for the formation of water pockets. To understand where these hydraulic barriers form, we write the subglacial hydraulic potential $\psi$ in meters water-equivalent (i.e. the hydraulic head) as

$$ \psi = \frac{p}{\rho_w g} + z_b, \qquad (1) $$
a

① Subglacial cavity formation

<figure><img src="image_10.png" /></figure>

② Subglacial cavity development

<figure><img src="image_11.png" /></figure>

③ Temporary blockage and filling of the cavity

<figure><img src="image_12.png" /></figure>

④ Outburst

<figure><img src="image_13.png" /></figure>

<figure><img src="image_14.png" /></figure>

Oberer Grindelwaldgletscher

<figure><img src="image_15.png" /></figure>

<figure><img src="image_16.png" /></figure>

**Fig. 6.** WPOFs originating from temporary blockage of subglacial channels. a) Schematic development of the blockage, linked to the partial collapse of subglacial cavities. b-d) Examples of hydrographs for three WPOFs in the Swiss Alps for which a temporary blockage of the subglacial channel seems likely. Note the different scales for both the x- and y-axes across the panels. The shaded areas separated by dashed lines and the numbering represent different phases of WPOF events and correspond to the numbers in panel a): 3) drainage cut-off linked to subglacial blockage and cavity filling, and 4) sudden, fast and sharp increase in discharge followed by fast decrease, linked to the outburst flood caused by sudden mechanical breach of the subglacial ice dam. The estimated baseflows, used to calculate the flood volumes, are indicated by the red dashed lines.
where $p$ is the water pressure, $g$ the gravitational acceleration, $\rho_w$ the density of water and $z_b$ the elevation of the bedrock (?). ? assumed that water pressure is equal to the overburden ice pressure, and thus suggested

$$ \psi \approx \psi_S = \frac{\rho_i}{\rho_w} (z_s - z_b) + z_b = \frac{\rho_i}{\rho_w} \left( z_s + z_b \left( \frac{\rho_w}{\rho_i} - 1 \right) \right), \quad (2) $$

where $\rho_i$ and $\rho_w$ are the ice and water density, respectively, and $z_s$ is the glacier surface elevation.

Figure ??a shows the conceptual case of a water pocket formed by a hydraulic barrier, with the hydraulic head $\psi_S$ defined in Equation ???. Note that this case example is computed by using real data presented in Figure ??b and described later in this section. The hydraulic barrier at the seal is caused by a rise in surface elevation along the general direction of englacial and subglacial water flow (note that in Equation ??, $\psi_S$ is mainly sensitive to surface elevation since $\rho_w/\rho_i - 1 \approx 0.09$). The maximum water pocket extent is controlled by the highest point of the hydraulic head situated at the hydraulic barrier (the seal) which in turn sets the height the water pocket can reach on the opposite shore $z_A$. Thus, the maximal hydraulic head which can be reached within the water pocket ($z_A + (\rho_i/\rho_w)h_A$) is the head at the seal ($h_A$ being the ice thickness at point A). Once that head is exceeded, the barrier breaks. As long as there is water input for the pocket to grow and the water within the pocket does not reach point A, the water pocket accommodates the new water by lifting the overlaying ice (see Appendix ?? for water pocket depth and volume calculation).

In Figure ??a, the water pocket is at the rupture point as the water pocket extends up to point A, i.e. to the point where the pocket's hydraulic head equals the head at the seal. At this point, the seal breaks by ice dam flotation and the reservoir empties through sheet flow and channel enlargement (similar to the outburst mechanism of ice-marginal and subglacial lakes; ???). Note that in Fig. ??a, the hydraulic head at A is higher than the glacier surface at the minimum hydraulic head, meaning that in case of a hydraulic connection between the surface and the water pocket, a pond could form at the surface. If no such connection exists, the ice roof of the water pocket can lift during the filling phase, which should be visible from the surface. Similarly, one should be able to observe surface lowering and potentially the appearance of circular crevasses during the drainage phase (e.g.?). After an outburst, local minima of hydraulic head can still remain. This could explain the repetitive occurrence of WPOFs for some of the glaciers in our inventory.

The WPOFs at Glacier d'Orny in 1920, Glacier du Mont-Miné in 1943, and Minstigergletscher in 2008 are suspected to have been caused by a hydraulic barrier similar to what is shown in Fig. ??a. At
Glacier d'Orny, ? suggested that the previous glacier advance modified the subglacial topography at the glacier tongue, creating an "ice dam for the subglacial streamflow until the water pressure exceeded the dam's resistance". At Minstigergletscher (see Fig. ??), the presence of debris at the glacier surface led to differential surface melt, locally thicker ice and, in turn, to a hydraulic barrier, i.e. steep gradients and a local depression in the hydraulic potential field. The water pocket was likely formed through the filling of existing subglacial voids near the location of the hydraulic barrier. ? also explained a possible water pocket formation at Mendel Glacier in the Sierra Nevada (California) by a local minimum in the hydraulic potential right below a topographic depression at the glacier surface (see her conceptual Fig. 9). The author hypothesised that the depression formed because of differential ice melt at the ice-debris boundaries, similar to the Minstigergletscher case discussed above.

Due to the typical nature of the en-/subglacial drainage system in alpine glaciers, a hydraulic barrier alone is not a sufficient requirement for a water pocket to form. For instance, if there is an existing englacial conduit that links the water reservoir located at the hydraulic barrier to the glacier surface, the extra water pressure may not lift the ice to form a water pocket but instead lead to the formation of a pond at the surface. Taken together, this means that a hydraulic barrier may not be impermeable enough to seal the upstream area sufficiently; in fact, this is probably the case at most such locations.

To analyse the potential presence of hydraulic barriers in Swiss glaciers, we mapped theoretical hydraulic barriers. We used the open-source package *WhereTheWaterFlows.jl* (see Code and data availability) to route the subglacial water formed by hydraulic barriers to the portal of all Swiss glaciers, and to fill the depressions in the subglacial hydraulic potential (Appendix ??). The necessary input data are glacier surface and bedrock topography. We used surface and bedrock digital elevation models (DEMs) from ?, which have a spatial resolution of 10 m and source dates from 2013 to 2018. *WhereTheWaterFlows.jl* does not account for temporal fluctuations in water pressure and is limited by the spatial resolution of the DEMs used. As a consequence, only relatively large potential subglacial water pockets sealed according to the hydraulic barrier mechanism are indicated by our approach, and small water pockets caused by local-scale topographic features (e.g. the water pocket at Minstigergletscher described above) cannot be resolved. For 17 out of the 37 glaciers with known WPOF events (i.e. 46%), our analysis identified current locations where water is likely to accumulate by a hydraulic barrier. Note that this result cannot be related to past events (i.e. before 2013) because we calculated the hydraulic potential with data on the present glacier extents and surface topography. Out of the 1400 glaciers we examined, our analysis identified hydraulic
<figure><img src="image_17.png" /><figcaption>**Fig. 7.** a) Cross section of a hypothetical water pocket caused by a hydraulic barrier at Glacier de la Plaine Morte (Switzerland). *z*<sub>s</sub>, *z*<sub>b</sub> and *ψ*<sub>S</sub> are the surface elevation, the bedrock elevation and the hydraulic head, respectively, along the profile (red arrow) shown in panel b). “A” is the highest point of the water pocket before its rupture, its elevation is controlled by the hydraulic head at the seal. b) Orthophoto of Glacier de la Plaine Morte from the Federal Office of Topography (swisstopo) with contour lines of *ψ*<sub>S</sub> (black) and water pocket depth (blue shadings). The glacier outline is from the SGI2016 (?). c) Topographic map from swisstopo and location of Glacier de la Plaine Morte in Switzerland.</figcaption></figure>

barriers in only 93 of them (i.e. 6%). Fig. ?? shows an example of a possible water pocket formation by
a hydraulic barrier at Glacier de la Plaine Morte (Switzerland): the water pocket would form at the local
minimum of the Shreve hydraulic potential (Fig. ??b), which can be explained by the local depression
at the glacier surface (Fig. ??a). We note that even though this glacier experienced lake outbursts on a
regular basis (??), no WPOF was ever reported.

4.3 Water-filled crevasses

Another mechanism that could produce WPOF-type events is the emptying of water-filled crevasses. We argue that water-filled crevasses can be classified as water pockets when they are either fully englacial (as the left water-filled crevasse in Fig. ??) or relatively narrow at their top, making them more similar to englacial water bodies than to supraglacial ones (middle water-filled crevasse in Fig. ??). The water pocket rupture may occur when water pressure exceeds the confining ice pressure, initiating hydrofracturing (e.g. ??). This process can connect the water-filled crevasse to the main drainage system, leading to progressive enlargement of the subglacial channel and eventually resulting in flooding (see also Section ??).
In temperate ice, water-filled crevasses are not common because they are generally well connected to the englacial drainage system and are therefore less likely to retain water (?). Water can vertically penetrate through the entire ice column (??) because the water density is higher than the ice density and because the water pressure can overcome the cryostatic pressure, in turn allowing water in crevasses to reach the glacier bed (??). In 1920, a WPOF from water-filled crevasses was observed at the snout of Mellichgletscher (?). The WPOF at Tête Rousse in 1892 was explained by the formation of a supraglacial lake during a negative mass balance anomaly, and by the burial and isolation of this lake from the surface during the following years of positive mass balance (?). Although in that case the reservoir had developed from a supraglacial lake, the rupture mechanisms might have been similar to water-filled crevasses being isolated from the glacier surface. In 1904, a 22,000 m³ water-filled crevasse was artificially drained at Tête Rousse (?).

In cold ice, water-filled cavities can originate from the ice dynamic closure of supra-/englacial water channels or water-filled crevasses, both associated with decreases in surface melt and water flow at the end of the melt season. Such a cut-and-closure mechanism is well documented and recognized in the literature (e.g. ??) and is supported by numerical modelling (?). Water-filled crevasses can also propagate into cold ice by hydrofracturing (e.g. ??). In the Swiss Alps, ? observed an isolated water-filled crevasse of at least several hundred cubic meters in cold ice (-2°C) at Jungfraujoch (3467 m a.s.l., Bernese Oberland) in the 1950s. The authors attributed the origin of this water to melt water from the surface. ? found a 100 m³ water-filled cavity in the cold ice of Breithorn peak (Valais) at 4000 m a.s.l. ? discovered a pressurized and isolated water-filled cavity (of unmeasured volume) at a depth of 9 m in cold ice while drilling in Athabasca Glacier (Canada). Note that none of the water-filled reservoirs mentioned above have known outbursts, and that their volumes are relatively small. However, they illustrate the potential of englacial water storage in alpine cold ice. ? suggested that the water pocket at Glacier de Tête Rousse (France) discovered in 2010 (that did not cause an outburst) was connected with another, smaller water reservoir located upglacier, that was in fact a water-filled crevasse. In summer 2022, the partial break-off of Ghiacciaio della Marmolada (Italy) resulted in a catastrophic ice avalanche that claimed the lives of eleven climbers. ? as well as ? suggest that the glacier break-off was caused by high water pressure in a water-filled crevasse above a steep bedrock slope, and that the water-filled crevasse was presumably disconnected from the en-/subglacial drainage system due to cold ice (there were observations of water flowing out of the water-filled crevasses at the glacier surface before the detachment). The melt water volume produced in the catchment of the crevasse during seven weeks of abnormally high air temperature
before the event was estimated to 11'000 m³ (?), demonstrating the significant potential for water storage in crevasses. Similar to Ghiacciaio della Marmolada, the ice avalanche of Ghiacciaio Coolidge (Italy) in 1989 was also related to a water-filled crevasse prior to the collapse (??). Although the Marmolada and Coolidge events might rather be categorized as ice avalanches rather than WPOFs, the englacial water accumulation preceding the partial glacier break-off indicates the potential of water storage in water-filled crevasses of cold or polythermal alpine glaciers. Cold ice acts as a thermal barrier by preventing water to flow. Note that for the water pocket formation mechanism presented in this section, the cavity (i.e. the crevasse) is already existing and filled with water after its formation. This differs from the mechanism presented in the next section, also related to polythermal conditions but called thermal barrier, where the cavity formation is initiated by water pressure exceeding the ice overburden pressure.

## 4.4 Thermal barrier

We use the term "thermal barrier" to describe the water pocket formation mechanism in which a subglacial water reservoir develops beneath the cold-temperate transition surface (CTS) of a polythermal glacier (Fig. ??; see ?, for a review on the hydrology of polythermal glaciers). We hypothesize that at the onset of water pocket formation, the presence of an initial void is not necessary, but that some connection to the en-/subglacial drainage network upstream of the thermal barrier is sufficient. The growth of the water reservoir is then controlled by ice-creep and flotation caused by high water pressure. The latter is in turn controlled by the hydraulic potential and water input (?). If the local subglacial hydraulic potential along the CTS allows for water accumulation and if the water pressure is higher than the ice overburden pressure, the cavity will grow. This growth will continue until the ice dam is eventually in hydrostatic equilibrium. The reservoir rupture then happens when the water pressure exceeds a given threshold (likely higher than the ice overburden pressure, as would be sufficient for temperate ice, i.e. for ice that is not frozen to the bed), or when the ice temperature reaches the pressure melting point (since this allows the water to escape subglacially by channel enlargement, e.g. ?).

The best documented example for a water pocket formation and filing mechanism caused by thermal barrier is the one discovered in 2010 at Glacier de Tête Rousse at 3180 m a.s.l. (???). In the Tête Rousse case, the particular topoclimatic factors associated with the glacier's small size and low slope (large snow accumulation at the top due to avalanches, relatively low accumulation at the tongue, and a mean annual air temperature of around -3°C) caused a cold-based, firn-free "plug" at the glacier tongue, and firn-insulated
temperate ice in the accumulation zone (?). In the firn-free area, the melt water ran off superficially, thus preventing any heat transfer into the ice by refreezing and latent heat release. In addition, the very low ice flow velocities of the glacier prevented both the advection of temperate ice downstream and significant strain heating (note that high ice flow velocities would also likely deform the cavity and thus cause earlier drainage). This combination of factors was suggested to have caused the ice to be cold at such low elevations, resulting in the development of a water pocket over a period of about forty years (?).

This relatively long persistence of a water pocket is in strong contrast to what is suggested by the analysis of WPOF events in our inventory (see Section ??) and to the time scales involved in the alternatively proposed mechanisms of temporary blockage of subglacial channels and hydraulic barriers (see previous sections). Although the thermal barrier mechanism seems to be a particularly good explanation for water pocket formation in general, there is no direct evidence for this mechanism in our updated WPOF inventory for the Swiss Alps. A reason for this might be the long time scales related to the thermal changes themselves, as the slow evolution implies that the frequency of outbursts should be low, thus decreasing the likelihood for such events to be reported at all.

Assessing the likelihood for the thermal barrier mechanism to cause WPOFs in a given glacier would require knowledge about the englacial temperature distribution in that glacier. However, regional-scale data on such distributions is lacking (e.g. ?), making the assessment of this likelihood impossible at larger scales. This is unfortunate, as the long time periods over which water can accumulate also means that this type of WPOF has potentially large flood magnitudes.
<figure><img src="image_18.png" /><figcaption>**Fig. 8.** Schematic for a water pocket caused by a thermal barrier. "CTS" stands for cold-temperate transition surface. Below the firn and snow cover, the temperate ice is isolated from conductive cooling while the percolation and refreezing of water in the firn releases heat. In the firn-free area, conductive cooling is more effective, while melt water runs off superficially thus minimizing the heat transfer into the ice. The geometry of the water pocket is driven by the ice and water pressure, the latter being controlled by the subglacial hydraulic potential and the upslope water input.</figcaption></figure>
# 5 DISCUSSION

## 5.1 Implications of the meteorological analyses on the formation and the rupture of water pockets

The results of the meteorological analysis (Section ??) suggest that the destabilization and eventual rupture of water pockets in temperate glaciers are most often linked to short-term processes (i.e. processes that take a maximum of a few days), and more specifically to meltwater input. The rupture of water pockets caused by precipitation input, instead, seems to play only a secondary role, since WPOFs do not always occur together with or after intense precipitation. Nevertheless, in individual cases, precipitation might play a role in triggering WPOFs by saturating and destabilizing the englacial and subglacial drainage network (e.g. ?). The timing and intensity of the precipitation event might be more important for the triggering of WPOFs than the total water amount. This is because fast water input into the glacier drainage system can produce high water pressures, especially when the glacier's drainage capacity is limited (e.g. ??). Since the sub-daily precipitation intensity is not known for most of the WPOF events in our inventory, however, it is difficult to further investigate this relationship.

The formation processes of water pockets that typically occur within a few days suggest that the water predominantly fills pre-existing cavities. Indeed, the process of opening glacial cavities is relatively slow (e.g. ?), and is unlikely to occur within the time frame of a few days. Water pockets formed by short-term processes such as temporary blockages (see Sec. ??) are likely part of an active en-/subglacial drainage system, which we think is the most common case in temperate glaciers. Conversely, water pockets isolated from the drainage system may accumulate water over longer periods, and are thus more likely to occur under particular climatic and glacio-geomorphic conditions, such as the ones prevailing at the polythermal Glacier de Tête Rousse (see Section ??).

## 5.2 Perspectives for further research

The absence of direct observations of water pockets prior to and during outburst floods significantly limits our understanding of this phenomenon. By consequence, it is important for further research to prioritize the detection and monitoring of water pockets. For instance, while the water pocket at Glacier de Tête Rousse has been extensively studied (see Section ??), it represents only one of the four potential formation mechanisms proposed in this study, and, to our knowledge, there are no other direct observations of such
phenomena.

Detection of water bodies within glaciers can be achieved through geophysical techniques such as ground-penetrating radar (e.g. ??), surface nuclear magnetic resonance (e.g. ???), and seismic methods (e.g. ??). However, these methods typically entail substantial logistics and field effort, making them unsuitable for deployment across entire glaciers (the typical survey area of such methods is in the order of a few thousand square meters at most). Consequently, prior knowledge of potential water pocket locations is often necessary before deploying these methods. This study helps in providing such prior knowledge by proposing four distinct mechanisms for water pocket formation (Section ??). For instance, we suggest that hydraulic and thermal barriers are preferential locations for water pocket formation. Characterizing the subglacial hydraulic potential and the distributed englacial temperature through numerical methods (see below) could thus be useful before conducting local geophysical measurements.

Detection of water pockets forming due to hydraulic and thermal barriers could be observed through changes in surface elevation. As these two mechanisms involve cavity growth within the glacier, changes in surface elevation during filling and drainage (uplift and subsidence, respectively) could be detected using remote sensing techniques. This could include the differencing of DEMs, or the use of interferometric synthetic aperture radar (InSAR). ?, for example, detected drainage of three subglacial lakes (which we would call “water pockets” in this study since they were not formed by geothermal activity) in Greenland with volumes of $3.513 \times 10^6 \text{ m}^3$ using satellite stereo-images and DEM differencing. Although achieving the required temporal, horizontal, and especially vertical resolution at the regional scale remains challenging, obtaining such data at the glacier scale is possible by using uncrewed aerial vehicles for example (e.g. ??).

As another example, ? analysed patterns of eight InSAR-derived interferograms that indicate the surface subsidence over three subglacial lakes at Brady Glacier (Alaska, USA). The inferred changes in water volume ranged from 22'000 to 243'000 $\text{m}^3$ – a size that is pertinent to our definition of water pockets. Although the authors could not assess the potential for a sudden drainage because the large temporal spacing of their radar data caused loss of coherence, they suggested that the technique could enable the detection and characterization of englacial water bodies prior to their outburst. In this respect, a challenge is given by the temporal separation between data acquisition: it needs to be short enough to prevent decorrelation of the interferograms due to glacier motion, but large enough to capture the relatively small surface elevation changes related to water pocket growth.

A further challenge when aiming at inferring potential water pockets from surface elevation changes is
the presence of snow at the glacier surface. Indeed, the snow may limit detectability, as snow redistribution
by e.g. wind can be substantial and thus mask the changes. At Glacier de Tête Rousse, for example, sig-
nificant snow accumulation caused DEM differencing to fail in detecting the depression caused by artificial
water pocket drainage (?).

Water pockets formed due to thermal barriers might pose the highest flood potential due to their
possibly long period of water accumulation (?). Therefore, characterizing a glacier's thermal regime and the
presence of a potential cold-temperate transition surface is crucial. By accurately modelling the thermal
regime of glaciers at the regional scale, one could determine locations that could be favorable for the
formation of water pockets. In recent years, research on the thermal characterization of alpine glaciers has
progressed from simple 1D modeling (e.g. ??) to more complex three-dimensional models (e.g. ?). However,
the high computational demands of these physically-based models currently prevent their applicability at
the regional scale.

Understanding water pocket formation requires assessing a glacier's capacity to store water englacially and subglacially. Calculating the water balance at the glacier scale would in theory allow to quantify the stored water volume (see ?, for a review on glacier storage concepts). While water output can be constrained by discharge measurements at proglacial streams, water input is commonly estimated by using limited in-situ measurements and modeling of snowmelt, icemelt and rain (e.g. ?). Even more difficult to constrain are water losses or gains that happen through evaporation, condensation, sublimation or deposition. Although notable exceptions exist (e.g. ?), the possibility of using water balance calculations for estimating water storage amounts are thus limited, especially when considering that the water volumes involved in water pocket formation are expected to be small when compared to the overall water throughput of a glacier.

# 6 CONCLUSION

In this study, we reviewed the existing literature on alpine glacial water pockets and extended, updated
and analysed a pre-existing inventory of water pocket outburst floods (WPOFs) in Switzerland. We found
that the term "water pocket" has often been employed as an umbrella term for describing water originating
from unknown subsurface reservoirs during glacial outburst floods. We defined a glacial water pocket as
an englacial or subglacial water-filled cavity with a volume > 1000 m³, and explicitly excluded subglacial
lakes formed by geothermal heat.

Our inventory compiles information on 91 WPOFs for 37 glaciers, including 20 glaciers with repetitive
WPOFs and direct observations of the subglacial water reservoir for four cases. We suspect that a significant portion of WPOFs goes entirely unnoticed, due to the lack of observations and the difficulty in detecting smaller flood volumes at gauging stations positioned far downstream of glaciers.

Unquantified temporal biases in the observations prevent the interpretation of interannual trends for our inventory. In contrast, there is a clear seasonal signal, with the majority of WPOFs occurring between June and September. Among the 32 WPOFs for which gridded daily meteorological data was available, our analysis revealed anomalously high temperatures during the days prior to most events, and strong to heavy precipitation (> 10 mm per day) on the day of the event in 25% of the cases. This indicates that water pockets typically rupture in periods of high water input, either from melting or from intense precipitation. We suggest that this high water input is key for destabilizing the subglacial drainage system and thus trigger an outburst. Water pockets isolated from the drainage system seem to exist too, and can accumulate water over time periods from years to decades. Such type of water pockets are more likely to form in polythermal glaciers, as inferred from studies that were conducted at Glacier de Tête Rousse (??). Our Swiss-wide analysis also indicates that glacier-wide glacio-geomorphic variables are insufficient to identify WPOF-prone glaciers. Rather, water pocket formation seems to be determined by small-scale local topography and processes, but the lack of precise location data in our inventory prevents any such analysis.

Based on the collected information, we propose four mechanisms for water pocket formation in alpine glaciers. In synthesis, these are: 1) temporary blockage of subglacial channels: ice collapses from the roof of a subglacial channel or cavity creates a temporary ice dam that traps water, and causes a sudden outburst when the dam fails. This seems to be the most frequent mechanism in our inventory. 2) Hydraulic barrier: water accumulates at a local minimum of the subglacial hydraulic potential, the surrounding ice acts as a dam until the water pressure exceeds the ice overburden pressure, this triggers an outburst through the subglacial drainage network. 3) Water-filled crevasses: isolated crevasses fill with water and reconnect to the drainage system via hydrofracturing, progressively enlarging subglacial channels and causing outbursts. 4) Thermal barriers: subglacial water is trapped at the cold-temperate transition surface of a polythermal glacier, leading to an outburst when either the local water pressure exceeds a threshold that is higher than ice overburden pressure or when the ice warms to the pressure-melting point. This mechanism, comprehensively documented for Glacier de Tête Rousse in the French Alps, was absent in our Swiss Alps inventory.
To further deepen our understanding of the mechanisms controlling water pocket formation and rupture, we encourage more field-based research, as the current lack of observations constitutes a crucial limitation. In particular, we propose observational techniques such as ground penetrating radar, surface nuclear magnetic resonance, seismics, digital elevation models differencing, interferometric synthetic aperture radar, water balance calculations, and ice temperature measurements combined with numerical modeling to locate water pockets. We believe that the distinct mechanisms for water pocket formation proposed in this study will help in narrowing the area of investigation, and that they provide a path forward when aiming at identifying glaciers that may host a water pocket. For instance, we suggest to study the development of subglacial cavities indicated by newly formed circular crevasses, to model the subglacial hydraulic potential to identify potential water accumulation zones, and to map the cold-temperate transition surface in polythermal glaciers. We hope that this review of the current knowledge will foster future research, ultimately contributing to predict water pocket formation and WPOFs occurrence, and thus helping in mitigating the risks that derive from them.

## 7 CODE AND DATA AVAILABILITY

The WPOF inventory presented in Section ?? can be accessed via https://github.com/christopheogier/WPOFs_CH.git. The distributed map of the theoretical subglacial water pocket volumes for all Swiss glaciers (Section ??) is available through ETH Zurich's Research Collection, DOI: 10.3929/ethz-b-000667509. The Julia programming language package WhereTheWaterFlows.jl is accessible at https://github.com/mauro3/WhereTheWaterFlows.jl. The version used here is v0.8.1 and archived at https://doi.org/10.5281/zenodo.8061564.

## 8 ACKNOWLEDGEMENTS

This project was financially supported by the Swiss National Science Foundation (grant nr. 212061). The contributions by M. Fischer were funded by the University of Bern, Switzerland. The contributions by M. Thibert, A. Gilbert, C. Vincent and O. Gagliardini were partially funded by the French ministry for Ecological Transition and Territorial Cohesion (Ministère de la Transition écologique et de la Cohésion des territoires) through its PAPROG program. The authors thank Dorde Masovic for his work on Figures ??, ??, and ???. The authors thank Felix Altrock, Sarah Lanz, Claudio Steffen, Flavia Zimmermann,
Eric Bardou, Daniel Devanthery, Raphaël Mayoraz, Martin Proksch and Nils Hählen for their contributions to the WPOFs inventory, and thank Alpiq SA for providing streamflow discharge data.

# A APPENDIX - THEORETICAL SUBGLACIAL WATER POCKET VOLUMES FOR ALL SWISS GLACIERS

We used the VAW-ETHZ package WhereTheWaterFlows.jl (WWF) to artificially fill the depressions in the subglacial hydraulic potential (i.e. to create artificial water pockets) and to route the subglacial water formed by hydraulic barriers to the portal of all Swiss glaciers. WWF is based on the D8 routing algorithm developed by ? and implements Shreve equations (?) for the hydraulic potential (see ???, for various application in glacier hydrology). We consider the predicted water pocket volumes as a proxy for potential subglacial water accumulation. The map of the theoretical distribution of subglacial water pockets for all Swiss glaciers can be downloaded from http://hdl.handle.net/20.500.11850/667509.

## A.1 Calculation of the subglacial hydraulic potential $\psi$

The subglacial hydraulic potential $\psi$, which drives the water flow, is defined by:

$$ \psi = p + \rho_w g z_b, \qquad (1) $$

where $p$ is the water pressure, $g$ the gravitational acceleration, $\rho_w$ the density of water and $z_b$ the elevation of the bedrock (?). ? assumes that water pressure is equal to the overburden ice pressure, and thus

$$ \psi_S = \rho_i g (z_s - z_b) + \rho_w g z_b \approx \psi, \qquad (2) $$

where $\rho_i$ is the ice density and $z_s$ is the elevation of the glacier surface.

The distributed bedrock elevation $z_b$ for all Swiss glaciers is obtained from digital elevation models derived from a combination of extensive ice thickness measurements from airborne ground-penetrating radar with glaciological modelling (?). The bedrock DEM has a spatial resolution of 10 m. The distributed glacier surface elevation $z_s$ is taken from the 2019 release of the swissALTI³D DEMs with 2 m resolution (Swiss Federal Office of Topography), which were resampled to a resolution of 10 m (?). On average, the data set represents the surface topography of the year 2015 (acquisition years span between 2007 and 2017) over glacierized areas. We then applied a moving-average filter to the surface elevation DEMs that is equal
to half of the ice thickness, or to 50 m as a minimum to prevent the impact of thin ice at the glacier margins. This is because the high-resolution swissALTI³D surface DEMs are sensitive to local features such as large crevasses or moulins. It also has a physical meaning because in reality the basal water pressure is also dampened by the ice thickness, such that small and local features at the ice surface don't necessarily have an impact on the basal conditions.

## A.2 Calculation of the water pocket volumes

Subglacial water pockets caused by hydraulic barrier develop in areas having a local minimum of the hydraulic potential as calculated with Equation ???. The subglacial water pocket depth is calculated by accommodating the water pressure to the ice overburden pressure. Thus, the subglacial water pocket depth $h_{wp}$ is given by

$$h_{wp} = \frac{\psi_f - \psi_S}{g \rho_w}, \qquad (3)$$

where $\psi_f$ is the filled Shreve potential and $\psi_S$ the Shreve potential. Note that $\psi_f = \psi_S(seal)$ in Figure ??b. The volume of the water pocket is then obtained by integrating the water pocket depth over its area with a spatial discretisation of 10 m due to the DEMs used (see subsection above). Note that in this simplistic treatment we assume that the water pocket fills slowly enough that the ice roof can uplift and is in hydrostatic equilibrium with the water in the reservoir.

## A.3 Limitations

The applied approach to calculate the theoretical subglacial water pocket volumes for all Swiss glaciers contains important uncertainties in the water pocket volumes themselves as well as their location. First, the algorithm was run using the glacier outlines from the SGI2016, which may not reflect the glacier surface topography at the time of older WPOF events (acquisition dates of source data used for glacier outlines in the SGI2016 vary from 2013 to 2018 (?)). The spatiotemporal resolution of existing DEMs reflecting the glacier surface topography for the entire Swiss Alps during the late 20th century or earlier (e.g. ?) is not sufficient for a reasonable application of the WWF tool. Second, we still observed artefacts of hydraulic potential values for some areas after application of the moving average filter because of the uncertainties in inferred bedrock. Third, flow routing in general is very sensitive to the spatial resolution of DEMs used and can show large uncertainties, especially for DEMs derived from Kriging methods or mass conservation (?).