How does an accumulating ice cover move upstream in a river?

An accumulating ice cover progresses upstream in a river as frazil ice pans gather at the leading edge of an existing ice cover. This process is driven by the interaction between the ice and the flow of the river.1

As the ice accumulates, it adds resistance to the flow and displaces a volume of water. This causes the river's depth to increase upstream, a phenomenon known as staging. The resulting increase in water level reduces the current velocity, which in turn allows the ice cover to progress into areas where the velocity was previously too high for ice formation.1

If the ice accumulation cannot withstand the forces exerted by the water flow and its own weight, it may thicken through a shoving process until it reaches a stable thickness. During cold periods, the freezing of the top layer can provide additional strength by distributing these forces to the shorelines. Conversely, if the mean velocity of the water exceeds approximately 60 centimetres (24 inches) per second, incoming ice floes will typically submerge and pass downstream rather than contributing to the upstream accumulation.1