Month: December 2018

Matthews’ 1992 geoecological model of vegetation succession within glacial forefields describes how following deglaciation the landscape evolves over time as the result of both biotic and abiotic factors, with the importance of each depending on the level of environmental stress within the system. We focus in this paper on how new understandings of abiotic factors and the potential for biogeomorphic feedbacks between abiotic and biotic factors makes further development of this model important. Disturbance and water dynamics are two abiotic factors that have been shown to create stress gradients that can drive early ecosystem succession. The subsequent establishment of microbial communities and vegetation can then result in biogeomorphic feedbacks via ecosystem engineering that influence the role of disturbance and water dynamics within the system. Microbes can act as ecosystem engineers by supplying nutrients (via remineralization of organic matter and nitrogen-fixation), enhancing soil development, either decreasing (encouraging weathering) or increasing (binding sediment grains) geomorphic stability, and helping retain soil moisture. Vegetation can act as an ecosystem engineer by fixing nitrogen, enhancing soil development, modifying microbial community structure, creating seed banks, and increasing geomorphic stability. The feedbacks between vegetation and water dynamics in glacial forefields are still poorly studied. We propose a synthesized model of ecosystem succession within glacial forefields that combines Matthews’ initial geoecological model and Corenblit et al.’s (2007) model to illustrate how gradients in environmental stress combined with successional time drive the balance between abiotic and biotic factors and ultimately determine the successional stage and potential for biogeomorphic feedbacks.

The paper has been accepted for publication in Progress in Physical Geography.

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.