Alexander M. Milner

Glaciers cover ∼10% of the Earth's land surface, but they are shrinking rapidly across most parts of the world, leading to cascading impacts on downstream systems. Glaciers impart unique footprints on river flow at times when other water sources are low. Changes in river hydrology and morphology caused by climate-induced glacier loss are projected to be the greatest of any hydrological system, with major implications for riverine and near-shore marine environments. Here, we synthesize current evidence of how glacier shrinkage will alter hydrological regimes, sediment transport, and biogeochemical and contaminant fluxes from rivers to oceans. This will profoundly influence the natural environment, including many facets of biodiversity, and the ecosystem services that glacier-fed rivers provide to humans, particularly provision of water for agriculture, hydropower, and consumption. We conclude that human society must plan adaptation and mitigation measures for the full breadth of impacts in all affected regions caused by glacier shrinkage.

The cryosphere in mountain regions is rapidly declining, a trend that is expected to accelerate over the next several decades due to anthropogenic climate change. A cascade of effects will result, extending from mountains to lowlands with associated impacts on human livelihood, economy, and ecosystems. With rising air temperatures and increased radiative forcing, glaciers will become smaller and, in some cases, disappear, the area of frozen ground will diminish, the ratio of snow to rainfall will decrease, and the timing and magnitude of both maximum and minimum streamflow will change. These changes will affect erosion rates, sediment, and nutrient flux, and the biogeochemistry of rivers and proglacial lakes, all of which influence water quality, aquatic habitat, and biotic communities. Changes in the length of the growing season will allow low‐elevation plants and animals to expand their ranges upward. Slope failures due to thawing alpine permafrost, and outburst floods from glacier‐ and moraine‐dammed lakes will threaten downstream populations. Societies even well beyond the mountains depend on meltwater from glaciers and snow for drinking water supplies, irrigation, mining, hydropower, agriculture, and recreation. Here, we review and, where possible, quantify the impacts of anticipated climate change on the alpine cryosphere, hydrosphere, and biosphere, and consider the implications for adaptation to a future of mountains without permanent snow and ice.

SUMMARY 1. This review examines the physical habitat and ecology of glacial rivers which have been relatively unstudied compared with rivers originating from other sources. 2. Typical glacial rivers have summer temperatures below 10°C, a single seasonal peak in discharge, which in the Northern Hemisphere typically occurs in July, a diel fluctuation in flow which usually peaks in late afternoon, and turbidity levels in summer that exceed 30 NTU. These variables contrast with those in snowmelt/rainfall streams, particularly in summer, and make conditions more extreme for the biota. 3. Where maximum temperatures are 2°C benthic invertebrate communities are dominated by Diamcsa (Chironomidae). Downstream, temperatures increase, channels become more stable and valley floors become older. Orthocladiinae (Chironomidae), Simuliidae, Baetidae, Nemouridae and Chloroperlidae become characteristic members of the invertebrate community. 4. Fauna may be displaced, or at least colonization delayed, by channel instability; the variable age structure of the valley floor will influence the faunal gradient, which may also be reset by the effects of tributaries, lakes and valley confinement. 5. We propose a qualitative model that outlines zoobenthic community gradients determined by two principal variables, water temperature and channel stability, as a function of distance downstream, or time since deglaciation.

Abstract Aquatic ecosystems in high latitude and altitude environments are strongly influenced by cryospheric and hydrological processes due to links between atmospheric forcing, snowpack/glacier mass‐balance, river discharge, physico‐chemistry and biota. In the current phase of global climate warming, many glaciers are shrinking. Loss of snow and ice‐masses will alter spatial and temporal dynamics in bulk basin runoff with important changes in the relative contributions of snowmelt, glaciermelt and groundwater to stream flow. Accordingly, altered water source contributions will be accompanied by changes to fluvial, solute, sediment and thermal regimes and, thus, channel stability and habitat. The projected reduction in sediment load, warmer water temperature and increased channel stability will drive significant shifts in the floral and faunal composition of glacier‐fed rivers. This paper hypothesizes a general increase in the richness and production of micro‐organisms, algae, macroinvertebrates and fish as glacier hydrological influence shrinks under a warmer climate. With reduced glacial influence, macroinvertebrate species trait diversity will increase with more organisms possessing larger body size, less specialized body shape and lower adult mobility. In larger river systems, potential reduction of meltwater inputs will have a significant influence on off‐channel habitats (e.g. side‐channels and sloughs) that depend on glacial runoff to sustain habitat availability and connectivity, particularly for fish. Some species such as cold stenothermic taxa (including some endemic macroinvertebrates) may be vulnerable to extinction and therefore gamma (regional) diversity will be reduced. These sensitive macroinvertebrate taxa may be important biological indicators of environmental change in glacierized river basins. Moreover, high climatic sensitivity and low human perturbation make glacially influenced river basins early indicator systems for identifying hydrological and ecological responses to climate change/variability. It is concluded that glacier shrinkage and associated changes in runoff amount and timing, water source contributions and physico‐chemical habitat will be a major driver of the future biodiversity of stream communities in cold environments. Research imperatives and future directions are proposed for investigation of glacier‐fed river hydroecology. Copyright © 2008 John Wiley & Sons, Ltd.