Meltwaters

Meltwaters are streams and rivers formed from melting snow and ice. They arise when temperatures rise and frozen material—such as seasonal snowpack, glaciers, and ice caps—releases liquid water that flows downslope, often combining in channels and lakes.

Meltwater originates from surface melt of snow, from glacier melt at the surface (supraglacial runoff), and from subglacial drainage that emerges at glacier margins or through moulins. Water may travel through melt channels on glacier surfaces, drain through englacial conduits, or flow over land once it leaves the ice.

Hydrologically, meltwater tends to have strong seasonal pulses, with peak flows in late spring and summer in many alpine and polar regions. It can transport large amounts of sediment (glacial flour) and nutrients, color rivers milky or turquoise, and interact with groundwater to feed streams after snowmelt ends.

Meltwaters support downstream ecosystems, supply freshwater for irrigation and hydropower, and shape landscapes by eroding bedrock and forming proglacial lakes, moraines, and outwash plains. They can also pose hazards, notably glacial lake outburst floods when lakes breach natural dams.

In a warming climate, meltwater regimes are shifting. Overall glacier mass loss may initially increase late-spring and summer runoff in some regions, but long-term trends point to reduced glacier volume and altered seasonal patterns. The distribution of meltwater is greatest in high mountain ranges and polar regions.

Meltwaters occur worldwide in places such as the European Alps, the Himalayas, the Andes, Alaska and central Asia, as well as in Greenland and Antarctica, where ice-sheet melt contributes to regional hydrology.