Why can ice be thinner over fast-flowing parts of a river?

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ice in lakes and rivers Heat flow through an ice cover (see text).

Ice cover on rivers can be significantly thinner in areas with higher water velocity because of the interaction between the ice and the water flowing beneath it. When water that is slightly above the freezing point flows under an established ice cover, the movement of the water either causes the undersurface of the ice to melt or retards the process of further thickening.1

Because the rate of this melting is proportional to both the water temperature and the velocity of the flow, ice in faster-moving sections of a river remains thinner than in areas where the water moves more slowly. These regions of thinner ice are often not visible from the surface, which can create dangerous conditions for anyone attempting to traverse the ice.1