How do hanging dams form beneath an ice cover in deep rivers?

In deep rivers, hanging dams form when frazil ice—small, disk-shaped ice particles that form in turbulent, supercooled water—is transported downstream from upstream reaches.12

Once this frazil ice reaches a section of the river with a fixed ice cover, it is carried beneath that cover. There, the particles deposit and accumulate, eventually building up into large, deep masses known as hanging dams. These formations can reach significant depths and may obstruct a large portion of the river's flow.1