Meltwater is water that is released from melting snow or ice. This includes meltwater from glacial ice and ice shelves over oceans. Meltwater is often produced during volcanic eruptions, and can cause dangerous lahars (landslides of wet volcanic debris). When meltwater pools on the surface rather than draining or flowing away, it forms pools known as melt ponds. Meltwater will often refreeze as the temperature drops. Meltwater can also collect or melt under the ice’s surface. Sub-glacial lakes are sometimes formed due to geothermal heat and friction.

Meltwater is the key source of drinking water for a large portion of the world’s population. It also is a key source of water for irrigation and hydroelectric plants. Some cities around the world have large lakes that collect meltwater to supplement water supply. Some well known cities that utilize meltwater are Canberra, Los Angeles, and Las Vegas.

Glacial meltwater comes from glaciers that have receded over long periods of time. Rivers can run through glaciers on their journey toward lakes. These striking blue lakes get their color from rock flour. Rock flour is a sediment that is transported through rivers to these lakes. This sediment comes from rocks that grind together underneath glaciers. The fine powder becomes suspended in water and refracts sunlight. The refraction gives it the milky turquoise appearance.

Meltwater acts as a lubricant that allows glaciers to flow across the landscape, albeit slowly. GPS measurements show that glacial movement is greatest during the summer months when meltwater levels are at their highest. Meltwater can also be an indication of abrupt climate change. The rapid vertical rising of an ice sheet along the Bindschadler Ice Stream, West Antarctica has suggested that there was a shift of a sub-glacial body of water that caused it. Sudden floods are capable of starting due to a destabilization in glacial bodies of water. These instabilities may also cause avalanches in snow packs. The chasms of the Purgatory Chasm State Reservation may have been caused by an instability in glacial meltwater lakes in the region. It is estimated that 40% of the world population could become affected by loss of glaciers due to global warming.

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