How does an upstream-moving ice cover raise the water level in a river?

When an ice cover accumulates and progresses upstream in a river, it raises the water level—a phenomenon known as staging—through two primary mechanisms.1

First, the ice cover adds significant resistance to the flow of the river. Second, the ice displaces a specific volume of water. Together, these effects increase the depth of the river upstream, which in turn reduces the flow velocity. This reduction in velocity enables the ice cover to continue progressing into areas where the current was previously too fast to allow for ice formation.1

It is important to note that this process is distinct from the formation of an ice jam, which occurs when the quantity of ice exceeds the river's transport capacity, often where the river slope changes or moving ice meets an intact cover. While both phenomena result in increased water levels and potential flooding, staging is specifically associated with the upstream progression of an accumulating ice cover.231