Why can ice jams form where a river flows into a lake?

Ice jams form at the point where a river flows into a lake because the moving ice encounters an intact ice cover.12

In hydrology, an ice jam occurs when the quantity of ice being transported downstream exceeds the river's transport capacity. While these blockages often form where a river's slope changes from steeper to milder, the transition into a lake creates a similar physical barrier. When moving ice meets the stable, stationary ice cover of a lake, it cannot continue its downstream progression, leading to an accumulation of stacked and clustered ice.12

This process is particularly common during spring breakups, when increased flow from snowmelt and rain breaks ice loose from riverbanks and carries it downstream. These jams can become thick enough to raise water levels, potentially causing localized flooding in the area behind the blockage.12