Why do floods occur on the Ob River while its lower course is still frozen?

Flooding on the Ob River while its lower course remains frozen is primarily caused by the formation of ice jams.

Because the Ob flows northward, the warmer southern reaches thaw earlier than the colder, more stable northern reaches. As spring arrives, snowmelt and increased runoff from the upper and middle sections of the river raise water levels and break the ice cover. This broken ice is transported downstream, where it encounters the still-frozen, intact ice cover of the lower river.123

When the quantity of moving ice exceeds the river's transport capacity, it accumulates and forms a blockage. This ice jam acts as a temporary dam, causing water to back up and flood the surrounding areas. In northerly flowing rivers like the Ob, this process is typical, as the upstream ice is freed first and moves toward the colder, more stable ice downstream.123