AlpWISE : ALPine Water, Ice, Sediment and Ecology

Climate change is driving an increase in river water temperatures, presenting challenges for aquatic ecosystems and water management. Many rivers are regulated by hydropower production, which alters their thermal regimes, causes short-term temperature fluctuations (thermopeaking) linked to flow variations, and whose future evolution under climate change remains uncertain. This study examines how the thermal regime of a peri-alpine regulated river could evolve under future climate scenarios using a high-resolution process-based model. Projections indicate that mean annual water temperatures may rise by up to 4 °C by 2080–2090 under RCP 8.5, with daily mean temperatures exceeding 15 °C for nearly half the year, raising ecological concerns. While these trends are comparable to those in unregulated rivers, river regulation introduces distinct spatial and seasonal patterns in climate change impacts. The reach with only a residual flow is particularly susceptible to warming due to limited discharge, whereas deep reservoir releases help moderate climate change impacts downstream of the dam and the hydropower plant. Furthermore, unlike in unregulated rivers where the strongest warming typically occurs in summer, climate change impacts in this regulated system are projected to be most pronounced in autumn and winter due to the thermal inertia of the reservoir. Indicators used to assess thermopeaking impacts remain largely unaffected by climate change, provided that hydropower operation remains unchanged. This study highlights that while regulation can exacerbate vulnerabilities to climate change, it also mitigates climate change impacts by influencing river temperature dynamics beyond thermopeaking alone. A copy of the paper is freely available here.

The formation and rapid expansion of glacial lakes following from climate change and rapid glacier recession in the Hindukush-Himalayan (HKH) region may increase the risk of glacial lake outburst floods (GLOFs). This paper develops a methodology for assess-ing the susceptibility of lakes (those ≥ 0.01 km² in size) to GLOF hazards. First, lakes were classified into six main types based on morphological features. Second, a multi-criteria approach (MCA) was applied, analyzing 13 factors across three GLOF-criteria: lake hydro-glacial characteristics, dam conditions and surrounding-triggers. The factors were ranked and weighted using an Analytical Hierarchy Process (AHP) and Pairwise Comparison Matrix (PCM) to drive a Susceptibility to Hazard Index (SHI). Lakes were categorized as high, medium or low susceptibility based on SHI values. The SHI- evaluations used past GLOF events from Afghanistan and the High Mountain Asia (HMA) dataset. Of 162 lakes assessed in Afghanistan, 36 showed high GLOF susceptibil-ity, and 54 with medium susceptibility. Smaller lakes (0.01−0.04 km²) dominated high and medium hazard classes, while larger lakes (0.1−0.5 km²) were few in these classes. This indicates smaller lakes may drive future GLOF risks in Afghanistan, where regular monitoring and fieldwork is important. These findings support hazard mapping and disaster risk reduction-measures for downstream-communities. A copy is freely available here.

Regulated rivers can experience sharp temperature variations induced by intermittent hydropower production (thermopeaking). To mitigate ecological impacts, dam operators need to assess the impacts of hydropeaking on stream temperature, and to test scenarios that might reduce them. While stream temperature modeling has been investigated in numerous studies, few have systematically assessed how integrated processes and their representation affect model performance, and models capable of capturing both sub‐hourly variations and long‐term thermal dynamics remain a challenge. Herein, a stream temperature model within the HEC‐RAS platform was used to model the thermal regime of a regulated river in Switzerland, with a 10‐min timestep over the annual time‐scale and for a 22‐km long reach; and for which we had installed a network of stream temperature sensors. While the initial model demonstrated an acceptable performance at the yearly scale (Mean Absolute Error: 0.78–2.10°C and Kling‐Gupta Efficiency: 0.55–0.85), this was not the case at the daily or
seasonal time‐scales. Two model corrections were found to be crucial; (a) the correction of potential incoming solar radiation for local shading; and (b) the representation of the heat flux linked to water‐sediment exchanges. With these two corrections, the annual performance improved (MAE: 0.48–0.83°C and KGE: 0.85–0.93) as did the daily and seasonal performance. Although physically based, the model required calibration, underscoring the importance of high‐quality in situ temperature data. The resulting model proves effective for practical applications in hydropower mitigation and river temperature management under complex flow regimes. A copy of the paper is freely available here.

The rapid retreat of mountain glaciers and the formation of new glacial lakes due to climate change pose considerable risks in mountain regions, especially where they increase the frequency and magnitude of glacial-lake outburst floods (GLOFs) and Jökulhlaups. This study presents an integrated inventory of glacial lakes in the Hindukush- Region of Afghanistan (HKA) for 2020 and the first multi-decadal assessment (1990–2020) of glacial lake changes. For 2020, we identified 2596 glacial lakes and supra-glacial ponds with an area of 96.8 ± 14.8 km2 (2362 lakes ≥0.003 km2 and 234 smaller lake/ponds <0.003 km2). This indicates an increase of 32% by number and 21% by area since 1990. We analyzed lake formation, expansion, decline and disappearance and found that patterns of change vary significantly between regions and as a function of altitude. 56% of lakes expanded, 20% shrunk, 17% were newly formed and 6% disappeared between 1990–2020. Glacial lakes in the HKA begin at ∼2980 m-asl, and the rate of increase of lake area was greatest for the elevation range 5300–5800 m-asl, during 2010–2020. Whilst lakes were generally increasing in area, those in the southwestern region of the HKA were decreasing, mainly because they were associated with smaller glaciers that were in a drier region. Smaller lakes (≤0.01–0.05 km2), particularly ice-contact proglacial lakes showed rapid increases, whereas larger lakes (>1.0 km2) tended to decrease in size. This study reveals the importance of considering geographical and altitudinal variation in glacial-lake formation and the role of smaller-lakes in GLOF-formation and water resources. A copy is freely available here.

Proglacial forefields are geomorphologically active zones with rivers that may buffer sediment connectivity between glaciers and downstream regions. To date, there are few studies of the magnitude of this process and the role of system‐internal perturbations, such as meltwater discharge rates, subglacial sediment supply and changing valley topography (e.g. valley slope, lateral accommodation space) following glacier retreat. Here, we use a physically‐based morphodynamic model, calibrated using continuous field‐collected data, to investigate the geomorphic response of these fluvial systems to varying topographical, sedimentological and hydrological boundary conditions. Results confirm the central role of the supply to transport capacity ratio in determining both proglacial stream geomorphic response and downstream sediment delivery rates, particularly for bedload particles; while suspended sediment transport is less affected by morphodynamic processes. Simulations also showed that the filtering effect is influenced by the lateral accommodation space and valley bottom slope. These observations not only suggest that the filtering varies between geomorphological settings, but also that the form of a valley (width, slope), as it is revealed following glacier retreat, will also impact filtering. These findings are important for understanding the future evolution of the longitudinal sediment connectivity in deglaciating catchments, in light of likely peak water and peak sediment. In the near‐future, the evolution of the proglacial margin filtering will have repercussion for the supply of sediment towards downstream regions, for the management of hydropower plants and for aquatic ecosystems. A copy of the paper if freely available here.

Rapid retreat of Alpine glaciers since the end of the Little Ice Age has increased the spatial extent of proglacial margins. Whilst the response of glaciers to climate warming is well-documented, the response of proglacial landscapes, especially regarding sediment transfer processes, is less understood. Sediment transport, through erosion and deposition, modifies the landscape and affects sediment connectivity. The latter may be described as structural (static, related to the configuration of the system) or functional (dynamic, resulting from time-space variation in sediment flux). This paper quantifies the functional connectivity of sediment flux for a recently deglaciated Alpine hillslope. Analysis of high-resolution digital elevation models is coupled to a morphological method to assesses the spatial pattern of sediment flux required to conserve mass over a seasonal and a multi-year time-scale. Results show that functional connectivity is strongly controlled by micro-scale topography on the hillslope that has developed during and since deglaciation. Functional connectivity is then a key factor determining the delivery of sediment to the valley bottom for eventual transport by the proglacial stream following glacier retreat. This effect is stronger at the seasonal time-scale than at the multi-year time-scale. A copy is freely available here.

Ongoing cryospheric change has modified sediment export from glacierized catchments substantially, with significant implications for ecosystems and downstream users, notably hydropower companies. Sediment is exported either as finer sediment in suspension or as coarser bedload with intermittent contact between sediment and the bed. To date, the difficulty in observing subglacial bedload transport limits the understanding of the physical processes associated with evacuating bedload compared with suspended load. We elucidate the factors controlling sediment export by inverting a physically-based numerical model of subglacial sediment production and sediment transport with suspended sediment and continuous bedload discharge records froman Alpine glacier. Comparable quantities of suspended sediment and bedload are exported, and model results suggest that both rely on the availability of sediment for transport. Yet, bedload export in subglacial channels also depends on particular hydraulic conditions, notably channel shape and hydraulic roughness. This makes exporting bedload-sized particles inefficient compared to fine-grained sediment. As a result, subglacial hydraulics should be explicitly considered when examining bedload export processes, and suspended and bedload transport should be considered separately. Inefficient bedload evacuation by melt water implies that glacial erosion may only continue when non-fluvial mechanisms evacuate sediment, such as sediment entrainment into the ice. A copy of the paper is freely available here.

Late Quaternary landscape evolution in tropical environments, such as Indonesia, remains poorly constrained due to limited prior studies and mineral properties that are challenging for luminescence dating. In this study, single- and multi-grain luminescence measurements of quartz and K-feldspar were explored for fill terrace deposits at the Kampar Kanan River, Indonesia. Our objective is to develop a chrono-stratigraphic framework that allows the reconstruction of late Quaternary fluvial morpho-dynamics, including climatic change. Quartz measurements were made using blue and green stimulation and single-aliquot regenerative dose (SAR) and double SAR protocols. However, as none of the quartz signals were fast component-dominated, they were not used for dating. Infrared-stimulated luminescence of multiple grains of K-feldspar at 50 ºC (IR50) and post-infrared infrared-stimulated luminescence at 225 ºC (pIR50IRSL225) yielded sufficiently bright signal intensities for dating, and ages were calculated using either the average dose (ADM) or minimum age model (MAM). The luminescence chronology based on fading corrected pIR50IRSL225 data yields ages from Marine Isotope Stage (MIS) 6 or earlier to MIS 1. The chrono-stratigraphy indicates that the river was likely aggradational during climate transitions from wet to dry with the deposition of more gravelly material, and erosional during colder periods when overbank deposition of fines may have been coincident with increased vertical river erosion due to a stronger monsoon.

A copy of the paper is freely available here.

Whilst confluence hydrodynamics are now very well known, their morphodynamics are less so. This is particularly true for bedload transport, including the trajectories followed by bedload supplied to a confluence by the main channel or the tributary, and their interactions. Field measurement of this phenomenon is currently difficult. Laboratory measurement allows study of gross effects, but not the detailed physics of the process. This paper applies a Lagrangian particle tracking model to a 90° discordant confluence, i.e. where the tributary enters the main channel at a height above the mainstream bed. The model was set up to represent the confluence of the Rhône and Avançon Rivers, Switzerland. The model is based upon a three-dimensional solution of the incompressible Navier-Stokes equations in the open-source toolbox OpenFOAM. It applies a Delayed Detached Eddy Simulation. Particle movement allows for coupled interactions between particles as well as between particles and the stream bed. Results show that as bedload arrives at the tributary mouth, there is size segregation. Some particles from the tributary fall rapidly to the mainstream bed and then travel along the main channel margin to the zone where bank-attached point bars commonly form. Others travel further into the main channel, notably if interactions between particles are allowed. The zone of maximum shear between the joining flows tends to have lower densities of particle tracks. There is sediment sorting at the junction with the very coarsest particles, due to interactions and momentum effects, and the very finest particles, due to greater ease of turbulent suspension, extending further into the main channel.

A copy of the paper is freely available here.

Study region

Zemu River, Upper Teesta River Basin, Eastern Himalaya

Study focus

In this study, a tree-ring chronology of Abies densa was used as a proxy to reconstruct a century-long May-June streamflow of the Zemu River. The reconstruction was carried out based on a scaling approach which explained 35 % of variance in observed streamflow and was varified by comparing with other regional reconstructions. Moving windowed Pearson correlation was performed to reveal the temporal influence of major climate forcings (e.g., ENSO) with the streamflow of Zemu River.

New hydrological insights for the region

Our study identified a strong negative relationship between tree-growth and observed May-June streamflow in the glacier-fed Zemu River. This counterintuitive, inverse relationship is likely due to the contribution of additional meltwater when conditions are dry and insulation over the glacier is high. The reconstructed streamflow record reveals several high- and low-flow periods above and below the mean value (+/- 1σ) and identifies 30 high-flow years and 33 low-flow years, including some historically recorded floods in 1927, 1968, 1980, 1982 and 1998, and of severe droughts in 1951, 1976 and 2017. Most recently, a positive association between reconstructed streamflow and the ENSO has become apparent, which we attribute to reduced penetration of the Indian Summer Monsoon during ENSO years, which reduces precipitation but maintains for longer warmer and drier conditions that allow glacier melt.

A copy of the paper is freely available here.