Anchor ice

Anchor ice is defined by the World Meteorological Organization as "submerged ice attached or anchored to the bottom, irrespective of the nature of its formation." It may also be called bottom-fast ice.Anchor ice is most commonly observed in fast-flowing rivers during periods of extreme cold, at the mouths of rivers flowing into very cold seawater, in the shallow sub or intertidal during or after storms when the air temperature is below the freezing point of the water, and the subtidal in the Antarctic along ice shelves or near floating glacier tongues, and in shallow lakes.

Anchor ice will generally form in fast-flowing rivers during periods of extreme cold. Due to the motion of the water, ice cover may not form consistently, and the water will quickly reach its freezing point due to mixing and contact with the atmosphere. Ice platelets generally form very quickly in the water column and on submerged objects once conditions are optimal for anchor ice formation. Anchor ice in rivers tends to be composed of numerous small crystals adhering to each other in small flocculent masses. Anchor ice in rivers can seriously disrupt hydro-electric power plants by significantly reducing flow or stopping turbines completely.

Another form of anchor ice may be observed at the mouths of Arctic rivers where fresh water seeps out of the river bed into the ocean up through the sediment. Anchor ice forms if the seawater is below the freezing point of the river water.

Shallow tundra lakes may feature anchor ice with a specific behavior. Lakes in the southwestern part of Nunavut, Canada typically freeze down to the bottom when the water level is low. On some cases spring meltwater flows into the lake under the ice cover, which becomes domed leaving a depressed "racetrack" ring around the shore where meltwater accumulates as well. The ice cover remains bottom-fast until the buoyancy force exceeds the freezing bond. At the latter moment the ice cover abruptly breaks off the bottom to form a flat sheet. In other cases the anchor ice becomes completely submerged into the meltwater and holes may be melted throughout the ice sheet. When the sheet finally lifts off the bottom, the meltwater accumulated at the surface is jetted through these holes with enough force to create small craters in the lake bottom where it is soft (sand or silt). This downward jet phenomenon was previously described for deltas into the Beaufort Sea, where they were caused by periodic tidal buoyancy of holed ice.

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