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Sediment export from glaciated basins involves complex interactions between ice flow, basal erosion and sediment transfer in subglacial and proglacial streams. In particular, we know very little about the processes associated with sediment transfer by subglacial streams. The Haut Glacier d’Arolla (VS, Switzerland) was investigated during the summer melt season of 2015. LiDAR survey revealed positive surface changes in the ablation zone, indicating glacier uplift, at the end of the morning during the period of peak ablation. Instream measures of sediment transport showed that suspended load and bedload responded differently to diurnal flow variability. Suspended load depended on the availability of fine material whereas bedload depended mainly on the competence of the flow. Interpretation of these results allowed development of a conceptual model of subglacial sediment transport dynamics. It is based upon the mechanisms of clogging (deposition) and flushing (transport/erosion) in sub-glacial channels as forced by diurnal flow variability. Through the melt season, the glacier hydrological response evolves from being buffered by glacier snow cover with a poorly developed subglacial drainage system to being dominated by more rapid ice melt with a more hydraulically efficient subglacial channel system. The resultant changes in the shape of diurnal discharge hydrographs, and notably higher peak flows and lower base flows, causes sediment transport to become discontinuous, with overnight clogging and late morning flushing of subglacial channels. Overnight clogging may be sufficient to reduce subglacial channel size, creating temporarily pressurised flow and lateral transfer of water away from the subglacial channels, leading to the late morning glacier surface uplift. However, without further data, we cannot exclude other hypotheses for the uplift.

The paper is in press with the journal Earth Surface Processes and Landforms and can be accessed here.

Improved water management strategies necessitate a solid understanding of environmental impacts associated with various flow release policies. Habitat suitability models use hydrodynamic simulations to generate weighted usable area curves, which are useful in characterizing the ecological suitability of flow release rules. However, these models are not conveniently run to resolve the hydrodynamics at the smaller scales associated with macroroughness elements (e.g., individual stones), which produce wakes that contribute significantly to habitat suitability by serving as shelter zones where fishes can rest and feed. In this study, we propose a robust environmental indicator that considers the habitat generated by the wakes downstream of stones and can thus be used to assess the environmental efficiency of flow release rules for impounded streams. We develop an analytical solution to approximate the wake areas behind macroroughness elements, and the statistical distribution of wake areas is then found using the derived distribution approach. To illustrate the concept, we apply our theory to four exemplary river streams with dispersed stones having different statistical diameter size distributions, some of which allow for an analytical expression of the weighted usable area. We additionally investigate the impact of spatiotemporal changes in stone size distributions on the usable area and the consequent threshold flows. Finally, we include the proposed environmental indicator to solve a multiobjective reservoir optimization problem. This exemplifies its practical use and allows stakeholders to find the most favorable operational rules depending on the macroroughness characteristics of the impounded stream.

The paper has been accepted for publication in Water Resources Research and can be accessed here.

Deltas are important coastal sediment accumulation zones in both marine and lacustrine settings. However, currents derived from tides, waves or rivers can transfer that sediment into distal, deep environments, connecting terrestrial and deep marine depozones. The sediment transfer system of the Rhone River in Lake Geneva is composed of a sublacustrine delta, a deeply incised canyon and a distal lobe, which resembles, at a smaller scale, deep?sea fan systems fed by high discharge rivers. From the comparison of two bathymetric datasets, collected in 1891 and 2014, a sediment budget was calculated for eastern Lake Geneva, based on which sediment distribution patterns were defined. During the past 125 years, sediment deposition occurred mostly in three high sedimentation rate areas: the proximal delta front, the canyon?levée system and the distal lobe. Mean sedimentation rates in these areas vary from 0·0246 m year?1 (distal lobe) to 0·0737 m year?1 (delta front). Although the delta front–levées–distal lobe complex only comprises 17·0% of the analysed area, it stored 74·9% of the total deposited sediment. Results show that 52·5% of the total sediment stored in this complex was transported toward distal locations through the sublacustrine canyon. Namely, the canyon–levée complex stored 15·9% of the total sediment, while 36·6% was deposited in the distal lobe. The results thus show that in deltaic systems where density currents can occur regularly, a significant proportion of riverine sediment input may be transferred to the canyon?lobe systems leading to important distal sediment accumulation zones.

The paper has been published in Sedimentology and can be accessed here.

In Alpine streams, humans have strongly modified the interactions between hydraulic processes, geomorphology and aquatic life through dams, flow abstraction at water intakes and river channel engineering. To mitigate these impacts, research has addressed both minimum flows and flow variability to sustain aquatic ecosystems. Whilst such environmental flows might work downstream of dams, this may not be the case for water intakes. Intakes, generally much smaller than dams, are designed to abstract water and to leave sediment behind. Sediment accumulation then results in the need to flush intakes periodically, often more frequently than daily in some highly glaciated basins. Sediment delivery downstream is then maintained through short duration floods with very high sediment loads. Here we tested the hypothesis that sediment flushing, and the associated high frequency of bed disturbance, controls in-stream habitat and macroinvertebrate assemblages. We collected macroinvertebrates over a 17-month period from an Alpine stream as well as a set of lateral unperturbed tributaries that served as controls. In contrast to established conceptual models, our results showed that the stream is largely void of life during summer, but that populations recover rapidly as the frequency of intake flushing falls in early autumn, producing richer and larger populations in winter and early spring. The recovery in autumn may be due to the recruitment of individuals from tributaries. We conclude that intake flushing in summer inverts expected summer-winter macroinvertebrate abundances, and questions the extent to which environmental flows in intake-impacted Alpine streams will lead to improvements in instream macrofauna unless sediment also is managed.

The paper can be accessed here (or email stuart.lane@unil.ch for a copy): https://www.sciencedirect.com/science/article/pii/S0048969718336246

Stuart Lane will serve on the Swiss National Science Foundation Research Council from the 1st January 2019, representing environmental science in the Division 4 (programmes)

Congratulations to Annegret Larsen who has been appointed to a Lectureship in Catchment Science at the University of Manchester, UK, from the 1st September 2018.

Congratulations to Chrystelle Gabbud who has been appointed environmental manager with Alpiq SA from 1 August 2018.

The group is part of a consortium led by Tom Battin at EPFL who has received four years of funding to study microbial biofilms and their implications for habitability and ecosystem evolution in glacial floodplain streams. Abbreviated to “ENSEMBLE” the focus of the project is how, in the presence of rapidly retreating glaciers, microbial biofilms respond and in turn impact the ecosystem evolution of glacial-floodplain systems. We will work at the interface of microbial ecology, vegetation ecology, hydrology and geomorphology to address the following questions:

(i) What are the ecological strategies and genomic underpinnings of biofilms that make them successful in colonizers in GFSs that emanate from glacier shrinkage?

(ii) Do microbial biofilms change, through ecosystem engineering, the geomorphodynamics of GFSs thereby increasing their habitability?

(iii) Does the interaction between microbial and geomorphic processes affect ecosystem heterogeneity in glacier floodplains?

We will be working primarily at the Otemma glacier in the Val de Bagnes (VS), but also the Rosengletscher (GR) and Zinal glacier, Val d’Anniviers (VS).

A new book, the Handbook of Critical Physical Geography, has been published by Stuart Lane, with colleagues Rebecca Lave and Christine Bierman.

Congratulations to Maarten Bakker who has been awarded an SNSF Early Post Doc Mobility grant to work at Grenoble Alpes University, France.