Glacial stream

A glacial stream (also called a glacier-fed stream or proglacial stream) is a stream or river whose flow includes meltwater from a glacier. Compared with streams fed mainly by rain, snowmelt or groundwater, glacial streams are typically colder, more turbid (cloudy with suspended sediment), more unstable in channel form, and more variable in discharge from hour to hour and season to season.^[1] These conditions make them demanding habitats, and the communities that live in them are correspondingly specialized.^[2]

Because their flow depends on ice, glacial streams respond directly to changes in climate. As glaciers thin and retreat, the share of ice melt in the water they carry changes, with consequences for the river's temperature, sediment load, flow pattern and wildlife.^[1]

Physical character

In a detailed study of the Ödenwinkelkees river in the Austrian Alps, researchers described the maximum water temperature at the sampling site as consistently below 2 °C, the water as highly turbid from suspended glacial sediment, and the channel as highly unstable.^[2]

A review of mountain-river studies reports that reaches dominated by ice melt have significantly lower mean water temperature, lower electrical conductivity (a measure of dissolved ions), lower channel stability, higher suspended-sediment concentrations and larger discharge fluctuations than reaches dominated by groundwater.^[1] Low conductivity reflects the fact that glacier meltwater has had little contact with soil and rock. In a Swiss Alpine catchment, conductivity at the glacier outlet in summer ranged from about 10 to 25 µS/cm and was strongly inversely correlated with discharge (Pearson r = −0.84), meaning that higher flows carried more dilute water.^[3]

Water sources and downstream change

A glacial stream usually carries water from three kinds of source: ice melt, snowmelt and groundwater, each with a distinct chemical signature and flow pattern.^[1] Ice-melt influence declines and groundwater influence rises with distance downstream from the glacier margin, as the glacier's share of the catchment shrinks.^[1] The result is a gradient of conditions along the river, and a mosaic of habitats across the glacier's forefield, the ground exposed by the retreating ice.^[1]

Flow regime

Glacial streams show strong daily and seasonal rhythms driven by melt. In the Swiss catchment studied by Müller and colleagues, early-season discharge was high but varied little over the day, which the authors attribute to a limited-capacity subglacial drainage system. By August, daily (diel) fluctuations were larger, as the retreat of the snow line reduced the buffering effect of snowpack.^[3]

Below the glacier, streams often spread across gravel plains called outwash plains. In the same study, water infiltrating from the stream into the aquifer beneath the plain was the dominant form of groundwater recharge, so the river and the aquifer exchange water strongly.^[3]

Outburst floods

Some glacial streams are subject to sudden, very large floods when a store of water held in or under the ice is released. These are called jökulhlaups (Icelandic: jökulhlaup, "glacier burst"). In Iceland, floods from the Grímsvötn subglacial lake in the Vatnajökull ice cap have recurred at intervals of four to six years since the 1940s, with peak discharges of 600 to 10,000 m³/s, durations of two to three weeks and total volumes of 0.5 to 3.0 km³.^[4]

Ecology

The cold, turbid and unstable conditions limit which organisms can live in glacial streams. The Ödenwinkelkees food webs contained only 13 to 22 taxa and had short food chains, with a mean length of 2.00 to 2.36 links. They were also highly connected, with a directed connectance of up to 0.19.^[2] The dominant macroinvertebrates were non-biting midges (family Chironomidae) of the genus Diamesa, which fed on detritus, diatoms and other chironomids. Members of the Diamesa latitarsis group often dominate Alpine rivers where water temperature does not exceed 2 °C.^[2]

Glacier-fed rivers also support multiple taxonomic groups, including rare and endemic species, and are of conservation interest for that reason.^[1]

Response to glacier retreat

Glacier retreat changes the proportions of ice melt, snowmelt and groundwater reaching a river. A review of the evidence concludes that this will alter the mosaic of aquatic habitats across glacier floodplains and threatens multiple endemic and rare species.^[1]

The timing of change in meltwater supply depends on the glacier and the region. Modelling of the Patagonian Andes (40–56° S) estimated that 34% ± 13% of glacier area had already passed its peak melt by 2020 (after which glacier melt contribution declines as ice is lost), and that 68% ± 21% of glacier area would lose more than half its current volume by 2100. Projected volume loss ranged from 46% ± 9% under a low-emissions scenario (SSP1-2.6) to 67% ± 11% under a high-emissions scenario (SSP5-8.5).^[5]

Within the outwash plain, glacier retreat exposes more ground for the river to rework, which may increase exchange between surface water and groundwater and change how water is stored and released.^[3]

Open questions

The response of macroinvertebrates to glacier retreat is well described, but information on biofilms, microinvertebrates and vertebrates is comparatively scarce. The authors of a 2017 review argue that better understanding of whole-river food webs would improve predictions of how rivers respond to deglaciation and help identify sensitive and threatened species.^[1] In the Patagonian modelling, the choice of reference climate dataset was the largest source of uncertainty in melt projections for most glacier area, which suggests that better climate records for poorly instrumented mountain regions would reduce uncertainty most.^[5]