anchor ice

Ice that forms on the bottom of a river or stream, attached to rocks, gravel, and other submerged objects — growing upward from the bed rather than downward from the surface. Anchor ice forms on clear, cold nights when the streambed radiates heat faster than the water above it, and it can lift rocks from the bottom as it builds and detaches. It is one of the stranger phenomena in river hydrology — ice that grows from below.