Highlights 39 • Presents one of the few multi-decadal records of coarse (sand and gravel) export from a 40 glaciated river basin 41 • Suggests that increasing sediment transport capacity does not explain interannual 42 variability in sediment export implying important variation in sediment supply 43 • Shows how connectivity develops in a glaciated basin in response to glacier recession 44 • Proposes that fluvial reworking of glacial till may reduce sediment transport rates and so 45 reduce sediment connectivity 46 47

ABSTRACT. This study shows how a new generation of terrestrial laser scanners can be used to investigate glacier surface ablation and other elements of glacial hydrodynamics at exceptionally high spatial and temporal resolution. The study area is an Alpine valley glacier, Haut Glacier d'Arolla, Switzerland. Here we use an ultra-long-range lidar RIEGL VZ-6000 scanner, having a laser specifically designed for measurement of snow-and ice-cover surfaces. We focus on two timescales: seasonal and daily. Our results show that a near-infrared scanning laser system can provide high-precision elevation change and ablation data from long ranges, and over relatively large sections of the glacier surface. We use it to quantify spatial variations in the patterns of surface melt at the seasonal scale, as controlled by both aspect and differential debris cover. At the daily scale, we quantify the effects of ogive-related differences in ice surface debris content on spatial patterns of ablation. Daily scale measurements point to possible hydraulic jacking of the glacier associated with short-term water pressure rises. This latter demonstration shows that this type of lidar may be used to address subglacial hydrologic questions, in addition to motion and ablation measurements.

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 pressurized 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. © 2018 John Wiley & Sons, Ltd.

The natural flow hydrological characteristics (such as the magnitude, frequency, duration, timing, and rate of change of discharge) of Alpine streams, dominated by snowmelt and glacier melt, have been established for many years. More recently, the ecosystems that they sustain have been described and explained. However, natural Alpine flow regimes may be strongly modified by hydroelectric power production, which impacts upon both river discharge and sediment transfer, and hence on downstream flora and fauna. The impacts of barrages or dams have been well studied. However, there is a second type of flow regulation, associated with flow abstraction at intakes where the water is transferred laterally, either to another valley for storage, or at altitude within the same valley for eventual release downstream. Like barrages, such intakes also trap sediment, but because they are much smaller, they fill more frequently and so need to be flushed regularly. Downstream, while the flow regime is substantially modified, the delivery of sediment (notably coarser fractions) remains. The ecosystem impacts of such systems have been rarely considered. Through reviewing the state of our knowledge of Alpine ecosystems, we outline the key research questions that will need to be addressed in order to modify intake management so as to reduce downstream ecological impacts. Simply redesigning river flows to address sediment management will be ineffective because such redesign cannot restore a natural sediment regime and other approaches are likely to be required if stream ecology in such systems is to be improved.

Alpine hydroelectric power exploitation often aims to increase the volume of water stored behind impoundments, which may be achieved through flow abstraction and lateral transfer to storage. Intakes are designed to separate water from sediment which accumulates in settling basins and may be flushed sometimes at subdaily frequencies in glaciated basins. In some countries (e.g., Switzerland) intakes drain a greater basin area than impoundments yet legislation designed to improve instream ecosystems impacted by hydropower has almost entirely ignored them. Some research suggests that such streams have exceptionally low abundance and diversity of macroinvertebrates for some kilometers downstream of the intake flushing at high frequency in summer, but that populations can recover rapidly as soon as flushing frequency decreases in early autumn. However, such patterns could also result from natural flow variability, sediment transport, and morphological change in glacier-fed streams. We combine field measurements with habitat modeling to assess the impacts of sediment flushing on macrozoobenthos as compared to what might be expected in a natural, hydromorphologically dynamic Alpine stream. We show that water abstraction in itself could improve habitat conditions because it increases the relative contribution of less turbid and groundwater/unregulated sources. However, intake flushing leads to short duration, sediment-laden flows that can destabilize substantial areas of the stream bed and cause rates of lateral displacement of habitat much greater than the possible response by macroinvertebrates. Our results challenge current emphasis on minimum flows in such streams and argue that much more emphasis needs to be placed on sediment management.