Glacier Outburst Floods Research Articles

GEOLOGICAL MARKERS OF THE UPPER PLEISTOCENE DEPOSITS IN MAIN RIVER VALLEYS OF WESTERN SIBERIA

During the second half of the 20th century by the end of the first quarter of the 21st century the problem of differentiation and correlation of the main rivers of Western Siberia (primarily the Ob and Yenisey) valleys sediments was intensively discussed. Traditionally, the regional Quaternary history of the river valleys was considered as the “terrace flight” paradigm. According to it in the Neogene-Quaternary time alluvial terraces formed by sequentially from the older upper to the younger lower ones. It was considered that terrace formed by neotectonics, paleoclimate dynamics changed the fullness of the rivers, advance of the sea and dereliction influenced on base of erosion and blocking river valley by northern glaciers. As a result, in outside glacier zone appeared barrier basins and periglacial alluvium. However, on the official maps of the Quaternary deposits the heights, age and even the number of terraces differ significantly. Nowadays a lot of data are contrary to theory about flight consistently carved cycloclimatic terraces and periglacial alluvium facies replacing the ice-dammed northern lakes in the south of Western Siberia. In the article it was performed the updated concept of the sedimental structure of the Upper Neopleistocene in the main valleys of the region and taking into account data on glacier outburst floods from intermountain area to the plain. Its intensive erosional and accumulative activity has so far been underestimated.

Geophysical reconstruction of the late Holocene proximal proglacial landsystem at Skeiðarársandur, southeast Iceland

ABSTRACTSandur plains are extensive sedimentary bodies formed in proglacial settings by glaciofluvial processes. Icelandic sandar have been hypothesised to be comprised of thick alluvial successions that provide critical insight into the processes that contributed to their formation and evolution. However, to‐date, most sandar research has focused on the analysis of sedimentary successions associated to topographically‐confined and small‐scale systems. These, however, do not capture the variety or scale of processes that influence sandar architecture. Therefore, detailed subsurface analysis of large‐scale and unconfined sandar is vital to understand how these systems respond to fundamental drivers, such as: (i) glacier oscillations, and (ii) episodic sediment flux from glacier outburst floods (aka. jökulhlaups). We report an extensive, low‐frequency (40 & 100 MHz) ground‐penetrating radar (GPR) survey of the ice‐proximal component of a large‐scale () active sandur in southeast Iceland. A bright and continuous reflection (PR1) is identified within all radargrams and provides a boundary between pre‐LIA and LIA to present‐day sedimentation. GPR data reveals a ~50 m thick ice‐proximal sediment wedge that is attributed to jökulhlaup and surge‐related glaciofluvial activity during the Little Ice Age (LIA). An approximate rate of deposition of 0.2–0.65 m has been calculated for the sediment wedge above PR1. Furthermore, we propose an extensive, sandur‐wide, pre‐LIA ice marginal limit of Skeiðarárökull, southeast Iceland, based on observations reported here and in previous work.

Long-period variability in ice-dammed glacier outburst floods due to evolving catchment geometry

Abstract. We combine a glacier outburst flood model with a glacier flow model to investigate decadal to centennial variations in outburst floods originating from ice-dammed marginal basins. Marginal basins can form due to the retreat and detachment of tributary glaciers, a process that often results in remnant ice being left behind. The remnant ice, which can act like an ice shelf or break apart into a pack of icebergs, limits a basin's water storage capacity but also exerts pressure on the underlying water and promotes drainage. We find that during glacier retreat there is a strong, nearly linear relationship between flood water volume and peak discharge for individual basins, despite large changes in glacier and remnant ice volumes that are expected to impact flood hydrographs. Consequently, peak discharge increases over time as long as there is remnant ice remaining in a basin, and peak discharge begins to decrease once a basin becomes ice-free. Thus, similar size outburst floods can occur at very different stages of glacier retreat. We also find that the temporal variability in outburst flood magnitude depends on how the floods initiate. Basins that connect to the subglacial hydrological system only after reaching flotation depth yield greater long-term variability in outburst floods than basins that are continuously connected to the subglacial hydrological system (and therefore release floods that initiate before reaching flotation depth). Our results highlight the importance of improving our understanding of both changes in basin geometry and outburst flood initiation mechanisms in order to better assess outburst flood hazards and their impacts on landscape and ecosystem evolution.

Declining discharge of glacier outburst floods through the Holocene in central Patagonia

Glacier outburst floods are a major hazard in glacierized catchments. Global analyses have shown reduced frequency of glacier floods over recent decades but there is limited longer-term data on event magnitude and frequency. Here, we present a Holocene palaeoflood record from the Río Baker (Chilean Patagonia), quantifying the discharge and timing of glacier floods over millennial timescales. A catastrophic flood of 110,000 m3/s (0.11 Sv) occurred at 9.6 ± 0.8 ka, during final stages of the Late Glacial Interglacial Transition, followed by five flood-phases coeval or post-dating Holocene neoglacials. Highest flood frequencies occurred at 4.3–4.4 ka, with 26 floods of minimum discharges of 10,000–11,000 m3/s, and 0.6 ka with 10 floods exceeding 4600–5700 m3/s. The largest modern outburst flood recorded surpassed ∼3810 m3/s. Thus glacier flood magnitude declines from the order of 0.1 to 0.01 Sv over the Early to Mid Holocene, and to 0.001 Sv in the instrumental record.

The roles of agriculture and climate in land degradation in southeast Iceland AD 1700–1900

ABSTRACT Past human land-use is increasingly recognized as a driver of ecosystem change in seemingly natural landscapes. Quantification of historical land use is therefore critical for assessing the degree of human impact and requires integration of geography, ecology and history. This paper examines the impact of climate change, land-use and exposure to international markets on the terrestrial ecology in Suðursveit, in southeast Iceland between AD 1700 and 1901. For this we use historical data concerning land-use, livestock and demography, and mapping of glacier advance, glacier outburst floods (jökulhlaups) and agricultural land. Change in land-use or population during the eighteenth century was insignificant. Population declined in the first two decades of the nineteenth century due to the advance of Breiðamerkurjökull glacier. After AD 1820, Suðursveit became part of international market for agricultural products, which facilitated population and livestock increase. This led to overgrazing and severe land degradation in the nineteenth century.

Glacier outburst floods reconstructed from lake sediments and their implications for Holocene variations of the plateau glacier Folgefonna in western Norway

A sudden release of large volumes of water during a glacier outburst flood (GLOF) is a major hazard worldwide. Here, we identify the sedimentary signature of glacial and non‐glacial processes, including GLOFs, based on lacustrine sediments from the distal glacier‐fed Lake Buarvatnet in western Norway. Historically documented GLOFs in 2002 CE and during the 1980s CE are identified in the 210Pb‐ and 14C‐dated sediments. These events have the same sedimentary signature as 12 earlier events throughout the Holocene interpreted to represent previous GLOFs in the catchment. The GLOFs are interpreted to have occurred during periods when the glacier extent was similar to the modern positions, and the events are thus used to pinpoint past positions of the glacier terminus and, hence, the equilibrium line altitudes (ELAs). The results indicate that the glacier Svartenutbreen, located at the eastern part of Folgefonna, had a similar size in 2002 CE as c. 8200–8300 cal. a BP, corresponding to the 8.2 ka event in the North Atlantic region. The regrowth of Sørfonna after the Holocene Thermal Optimum occurred at c. 6900 cal. a BP and Svartenutbreen was at modern size and extent in the periods c. 6400, c. 5450, c. 4850, c. 3850, c. 3550 and c. 1650 cal. a BP. Since 1650 cal. a BP, we infer that the glacier was larger than the 2002 CE glacier extent until 1910 CE when a GLOF occurred. Svartenutbreen has been retreating since 1910 CE, which led to the ice damming of the two historical GLOFs in the 1980s and 2002 CE separated by a glacier advance in the 1990s CE. The findings are discussed and compared to other regional glacier reconstructions in Norway, and emphasize the value of identifying and utilizing GLOFs as an indicator of past ELA variability.

Brief communication: Observations of a glacier outburst flood from Lhotse Glacier, Everest area, Nepal

Abstract. Glacier outburst floods with origins from Lhotse Glacier, located in the Everest region of Nepal, occurred on 25 May 2015 and 12 June 2016. The most recent event was witnessed by investigators, which provided unique insights into the magnitude, source, and triggering mechanism of the flood. The field assessment and satellite imagery analysis following the event revealed that most of the flood water was stored englacially and that the flood was likely triggered by dam failure. The flood's peak discharge was estimated to be 210 m3 s−1.

A global assessment of the societal impacts of glacier outburst floods

Glacier outburst floods are sudden releases of large amounts of water from a glacier. They are a pervasive natural hazard worldwide. They have an association with climate primarily via glacier mass balance and their impacts on society partly depend on population pressure and land use. Given the ongoing changes in climate and land use and population distributions there is therefore an urgent need to discriminate the spatio-temporal patterning of glacier outburst floods and their impacts. This study presents data compiled from 20 countries and comprising 1348 glacier floods spanning 10 centuries. Societal impacts were assessed using a relative damage index based on recorded deaths, evacuations, and property and infrastructure destruction and disruption. These floods originated from 332 sites; 70% were from ice-dammed lakes and 36% had recorded societal impact. The number of floods recorded has apparently reduced since the mid-1990s in all major world regions. Two thirds of sites that have produced >5 floods (n=32) have floods occurring progressively earlier in the year. Glacier floods have directly caused at least: 7 deaths in Iceland, 393 deaths in the European Alps, 5745 deaths in South America and 6300 deaths in central Asia. Peru, Nepal and India have experienced fewer floods yet higher levels of damage. One in five sites in the European Alps has produced floods that have damaged farmland, destroyed homes and damaged bridges; 10% of sites in South America have produced glacier floods that have killed people and damaged infrastructure; 15% of sites in central Asia have produced floods that have inundated farmland, destroyed homes, damaged roads and damaged infrastructure. Overall, Bhutan and Nepal have the greatest national-level economic consequences of glacier flood impacts. We recommend that accurate, full and standardised monitoring, recording and reporting of glacier floods is essential if spatio-temporal patterns in glacier flood occurrence, magnitude and societal impact are to be better understood. We note that future modelling of the global impact of glacier floods cannot assume that the same trends will continue and will need to consider combining land-use change with probability distributions of geomorphological responses to climate change and to human activity.

Genesis and provenance of a new Middle Pleistocene diamicton unit at Happisburgh, NE Norfolk, UK

Glacigenic deposits at Happisburgh, NE Norfolk, record the earliest known expansion of glaciers into lowland eastern England during the Quaternary. The sequence comprises two regionally extensive till units, the Happisburgh Till and Corton Diamicton members of the Happisburgh Glacigenic Formation, deposited during separate ice advances, and intervening glacilacustrine and outwash deposits laid down during ice-marginal retreat. During 2012, a new diamicton unit was discovered within the intervening sorted sediments and its significance is outlined here. Sedimentological and structural evidence suggests, tentatively, that the diamicton forms a small debris fan generated subaerially by a series of water-saturated hyperconcentrated or debris flows. The precise trigger mechanism for these flow deposits remains unclear, but may relate to seasonal melting of surface or buried ice followed by mass-movement, or to more abrupt geological events including periods of intense rainfall, moraine dam failure or a glacier outburst flood.

Assessment of hydro-morphodynamic modelling and geomorphological impacts of a sediment-charged jökulhlaup, at Sólheimajökull, Iceland

Understanding of complex flood-riverbed interaction processes in large-scale (field) outburst floods remains incomplete, not least due to a lack of well-constrained field data on hydraulics and sediment transport, but also because consensus on an appropriate model framework has yet to be agreed. This study presents a novel full 2D hydro-morphodynamic model containing both bedload and suspended load capability. Firstly, the model design is presented with an emphasis on its design to simulate rapidly-varied sediment-laden outburst floods and also the associated geomorphological impacts. Secondly, the model is applied to a large-scale (field) glacier outburst flood or ‘jokulhlaup’ at Solheimajokull, Iceland. For this real-world event, model scenarios with only water and with inclusion of sediment with different parameter setups were performed. Results indicated that grain size specifications affected resultant geomorphological changes, but that the sensitivity of the simulated riverbed changes to the empirical bedload transport formulae were insignificant. Notably, a positive feedback occurred whereby the jokulhlaup led to significant net erosion of the riverbed, producing an increase in flow conveyance capacity of the river channel. Furthermore, bulking effects of sediment entrainment raised the peak discharge progressively downstream, as well as the flood volume. Effects of geomorphological changes on flood water level and flow velocity were significant. Overall, despite the increased computational effort required with inclusion of sediment transport processes, this study shows that river morphological changes cannot be ignored for events with significant in-channel erosion and deposition, such as during outburst floods.

Sedimentary and structural evolution of a relict subglacial to subaerial drainage system and its hydrogeological implications: an example from Anglesey, north Wales, UK

Subglacial drainage systems exert a major control on basal-sliding rates and glacier dynamics. However, comparatively few studies have examined the sedimentary record of subglacial drainage. This is due to the paucity of modern analogues, the limited recognition and preservation of upper flow regime deposits within the geological record, and the difficulty of distinguishing subglacial meltwater deposits from other meltwater sediments (e.g. glacier outburst flood deposits). Within this study, the sedimentological and structural evolution of a subglacial to subaerial (ice-marginal/proglacial) drainage system is examined. Particular emphasis is placed upon the genetic development and preservation of upper flow regime bedforms and specifically recognising them within a subglacial meltwater context. Facies are attributed to subglacial meltwater activity and record sedimentation within a confined, but progressively enlargening, subglacial channel system produced under dune to upper flow regime conditions. Bedforms include rare large-scale sinusoidal bedding with syn-depositional deformation produced by current-induced traction and shearing within the channel margins. Subglacial sedimentation culminated with the abrupt change to a more ephemeral drainage regime indicating channel-abandonment or a seasonal drainage regime. Retreat of the ice margin, led to the establishment of subaerial drainage with phases of sheet-flow punctuated by channel incision and anastomosing channel development under diurnal, ablation-related, seasonal discharge. The presence of extensive hydrofracture networks demonstrate that proglacial groundwater-levels fluctuated markedly and this may have influenced later overriding of the site by an ice stream.

Huss Receives 2013 Cryosphere Young Investigator Award: Citation

Matthias Huss is an outstanding young glaciologist who completed his Ph.D. in 2009 at the Swiss Federal Institute of Technology (ETH) in Zurich, Switzerland, and currently has a joint appointment there and at the University of Fribourg. During his short scientific career, Matthias has made substantial and innovative contributions to a broad range of topics in glaciology, including modeling and projection of glacier mass and runoff changes, glacier outburst floods, glacier dynamics, and climate‐glacier interactions. His work ranges from local‐scale to regional‐ and global‐scale studies. Of note, he developed a new cutting‐edge and already widely used parameterization to model glacier retreat, which fills the gap between complex ice flow models and simple scaling methods. He also computed ice thickness distribution maps of all 200,000 glaciers in the world based on principles of flow dynamics. This unique data set has many applications, of which perhaps the most significant is the estimation of the total volume of stored ice, a quantity that is critical for projections of sea level rise and water resources during the 21st century.

Landscape reaction, response, and recovery following the catastrophic 1918 Katla jökulhlaup, southern Iceland

AbstractOne of the largest recorded glacier outburst floods (jökulhlaups) occurred in 1918, generated by the last major subglacial eruption of Katla volcano in southern Iceland. Using digitized historical topographic surveys and field observations from the main proglacial outwash plain (Mýrdalssandur), we document the reaction of Mýrdalssandur to the 1918 event and subsequent response and recovery. Our analysis highlights the longevity of elevated topography, over the recovery period, and the complete reorganization of the main perennial meltwater channel system, both of which will affect and condition the flow routing and impact of future jökulhlaups. The jökulhlaup deposited approximately 2 km3 of sediment onto Mýrdalssandur immediately after the event and extended the coastline by several kilometers. However, 80% of this material by volume has since been removed by surface and subsurface water flow on the main sandur and by marine reworking at the coast. By 2007, the surface elevation at specific locations on the outwash plain and the position of the coastline were similar to those in 1904, indicating near‐complete recovery of the landscape. Despite this, the Mýrdalssandur coastline has experienced net advance over the past 1000 years. Using our calculated characteristic landscape response and recovery values following the 1918 event (60 years and 120 years) we deduce that the landscape has been in a dominant state of transience, with regard to forcing frequency and timescale of recovery, over the past 1000 years, which has facilitated long‐term landscape growth.

The role of multiple glacier outburst floods in proglacial landscape evolution: The 2010 Eyjafjallajokull eruption, Iceland

The 2010 Eyjafjallajokull eruption in Iceland provided a unique opportunity to quantify the evolution of proglacial geomorphology during a series of volcanogenic jokulhlaups (glacial outburst floods) (>140 events). Time-lapse imagery and repeat terrestrial laser scans before and directly after the eruption show that the jokulhlaup of 14 April 2010 composed 61% of the 57 × 106 m3 total discharge of the combined events, and had the highest peak discharge for the two main flood events, but only deposited 18% of the total volume of sediment in front of Gigjokull glacier. The majority of sediments (67% of a total volume of 17.12 × 106 m3) were deposited by the 15 April 2010 jokulhlaup, and this was followed by extensive reworking by a series of smaller jokulhlaups over the following 29 days that deposited 15% of the total sediment. The geomorphological and sedimentary signatures of the two largest jokulhlaups associated with the onset of the eruption have either been reworked by later floods or are buried by later flood deposits. Consequently, the ice-proximal, posteruption landscape cannot be used to reconstruct the characteristics or magnitudes of either of the two largest jokulhlaups. The findings support a complex-response model in which peak discharge and the bulk of the sediment transported is decoupled by changing routing mechanisms and water:sediment ratios during the eruption.

Relationship between recent fluctuations of glacier snouts and outburst floods of glacier-dammed lakes in north western Karakoram Mountains, Pakistan

Relationship between recent fluctuations of glacier snouts and outburst floods of glacier-dammed lakes in north western Karakoram Mountains, Pakistan

Particle Fabric and Sedimentation of Structureless Sand, Southern Iceland

Abstract The interpretation of structureless sand units, in terms of flow character and associated depositional mechanism, is notoriously difficult to deduce from localized outcrop, because such units offer no macroscopic features with which to make distinctions of flow parameters. This investigation assesses the significance of directional properties of particle-fabric samples taken both vertically and horizontally within structureless sand units deposited by jokulhlaups (glacier-outburst floods) in southwest Iceland, known to be macroturbulent phenomena based on eyewitness accounts and inferences made from the large-scale sedimentary architectures of the deposits. Given this framework, we use particle fabric to verify flow character and depositional mechanism, highlighting its potential application to deposits where prior knowledge of conditions is unknown. Vertical and horizontal trends in particle fabric indicate a causative mechanism that is manifested both spatially and temporally within visually structureless units. This data indicates sedimentation onto an aggrading bed beneath a turbulent flow, which conforms to the documented turbulent character of the jokulhlaup. We demonstrate that cryptic information related to flow parameters can be gleaned from visually structureless sand units through detailed particle-fabric analysis, and clarify aspects of hyperconcentrated-flow behavior and sedimentation.

Late Ordovician climbing-dune cross-stratification: a signature of outburst floods in proglacial outwash environments?

Climbing dune-scale cross-statification is described from Late Ordovician paraglacial successions of the Murzuq Basin (SW Libya). This depositional facies is comprised of medium-grained to coarse-grained sandstones that typically involve 0·3 to 1 m high, 3 to 5 m in wavelength, asymmetrical laminations. Most often stoss-depositional structures have been generated, with preservation of the topographies of formative bedforms. Climbing-dune cross-stratification related to the migration of lower-flow regime dune trains is thus identified. Related architecture and facies sequences are described from two case studies: (i) erosion-based sandstone sheets; and (ii) a deeply incised channel. The former characterized the distal outwash plain and the fluvial/subaqueous transition of related deltaic wedges, while the latter formed in an ice-proximal segment of the outwash plain. In erosion-based sand sheets, climbing-dune cross-stratification results from unconfined mouth-bar deposition related to expanding, sediment-laden flows entering a water body. Within incised channels, climbing-dune cross-stratification formed over eddy-related side bars reflecting deposition under recirculating flow conditions generated at channel bends. Associated facies sequences record glacier outburst floods that occurred during early stages of deglaciation and were temporally and spatially linked with subglacial drainage events involving tunnel valleys. The primary control on the formation of climbing-dune cross-stratification is a combination between high-magnitude flows and sediment supply limitations, which lead to the generation of sediment-charged stream flows characterized by a significant, relatively coarse-grained, sand-sized suspension-load concentration, with a virtual absence of very coarse to gravelly bedload. The high rate of coarse-grained sand fallout in sediment-laden flows following flow expansion throughout mouth bars or in eddy-related side bars resulted in high rates of transfer of sands from suspension to the bed, net deposition on bedform stoss-sides and generation of widespread climbing-dune cross-stratification. The later structure has no equivalent in the glacial record, either in the ancient or in the Quaternary literature, but analogues are recognized in some flood-dominated depositional systems of foreland basins.

Landforms and landscape evolution in the Skardu, Shigar and Braldu Valleys, Central Karakoram

The Central Karakoram, which includes K2 in Pakistan, is one of the most rapidly rising areas on Earth and exhibits complex topography and extreme relief. Impressive valley fills and glacial landforms are present throughout the valleys. The dynamics of landscape evolution of the region are currently not well understood. Consequently, the landforms were mapped and assessed in the Skardu, Shigar, and Braldu valleys, to elucidate the spatio-temporal scale dependencies of surface processes active in the region. These valleys were examined using geomorphic field methods, remote sensing, geomorphometry, and terrestrial cosmogenic nuclides (TCNs) surface exposure dating. The glaciers in this region have oscillated considerably throughout the Late Quaternary, and four glacial stages have been recognized including at least six glacial advances. Surface processes readjusted after glacier retreat, and ubiquitous mass movements and catastrophic landsliding transported material from steep slopes to valley bottoms, while glaciofluvial meltwater and glacier outburst floods redistributed sediment down valley. Glacier geochronology and late Holocene ages of the outburst flood deposits indicate that landscape evolution has been dominated by glaciation and paraglaciation during the late Quaternary.

Integrated research on mountain glaciers: Current status, priorities and future prospects

Mountain glaciers are sensitive probes of the local climate, and, thus, they present an opportunity and a challenge to interpret climates of the past and to predict future changes. Furthermore, glaciers can constitute hazards, including: glacier outburst floods; changes in the magnitude and timing of runoff in the mountains and adjacent regions; and, through worldwide loss of glacier ice, a global rise in sea level. To understand and ultimately to predict the dynamics and nature of climate and associated glacial and hydrological changes requires an integrated approach with communication and collaboration among glaciologists, glacial geologists, atmospheric scientists, geomorphologists, geochronologists, and tectonists. Current strategies of research are evolving towards integrating research on mountain glaciers to address key scientific, socio-economic and political issues. Given the rapid birth and growth of new technologies and tools with which to study glaciers and glacial landscapes, this community stands poised to address many of these challenges in the near future. The key challenges that must be met soon include: 1) determining the spatial–temporal pattern of fluctuations of mountain glaciers from the last glacial cycle through the present; 2) relating historical and past fluctuations in glaciers to variability in the primary features of ocean-atmospheric circulation; 3) identifying important but poorly understood processes controlling the motion and erosion of glaciers; 4) developing and expanding the application of numerical models of glaciers; 5) modeling the evolution of mountain landscapes in the face of repeated glaciation; 6) examining the climate and the balance of energy and mass at the surface of glaciers; 7) characterizing the role of intrinsic climate variability on glacier variations; and 8) predicting the distribution, sizes, and nature of glaciers in the future. While these ambitious goals are achievable and the research tools exist, success will require significant bridging between the existing research communities involved and ambitious integration of research on mountain glaciers.

Wood-mediated geomorphic effects of a jökulhlaup in the Wind River Mountains, Wyoming

A jokulhlaup burst from the head of Grasshopper Glacier in Wyoming's Wind River Mountains during early September 2003. Five reaches with distinct sedimentation patterns were delineated along the Dinwoody Creek drainage. This paper focuses on a portion of the jokulhlaup route where erosion of the forested banks created 16 large logjams spaced at longitudinal intervals of tens to hundreds of meters. Aggradation within the main channel upstream from each logjam created local sediment wedges, and the jams facilitated overbank deposition during the jokulhlaup. Field surveys during 2004 and 2006 documented logjam characteristics and associated erosional and depositional features, as well as initial modification of the logjams and flood deposits within the normal seasonal high-flow channel. Overbank deposits have not been altered by flows occurring since 2003. Field measurements supported three hypotheses that (i) logjams present along the forested portions of the jokulhlaup route are larger and more closely spaced than those along adjacent, otherwise comparable stream channels that have not recently experienced a jokulhlaup; (ii) logjams are not randomly located along the jokulhlaup route, but instead reflect specific conditions of channel and valley geometry and flood hydraulics; and (iii) the presence of logjams facilitated significant erosional and depositional effects. This paper documents a sequence of events in which outburst floodwaters enhance bank erosion and recruitment of wood into the channel, and thus the formation of large logjams. These logjams sufficiently deflect flow to create substantial overbank deposition in areas of the valley bottom not commonly accessed by normal snowmelt peak discharges, and through this process promote valley-bottom aggradation and sediment storage. Changes in the occurrence of glacier outburst floods thus have the potential to alter the rate and magnitude of valley-bottom dynamics in these environments, which is particularly relevant given predictions of worldwide global warming and glacial retreat. Processes observed at this field site likely occur in other forested catchments with headwater glaciers.