How do ice jams form during the Ob River’s spring thaw?

Ice jams on the Ob River occur during the spring thaw as a result of the river's northward flow and the timing of its breakup.123

Because the Ob flows toward the north, the spring thaw begins in its warmer, upstream southern reaches while the downstream northern sections remain frozen and stable. As snowmelt and runoff increase the river's flow, the water level rises and breaks the upstream ice cover loose. This broken ice is then transported downstream by the current.231

When this moving ice encounters the colder, intact ice cover in the lower reaches, it cannot be transported further, leading to the formation of ice jams. These jams can build to significant thicknesses, obstructing the river and causing water levels to rise, which often results in flooding. On the Ob, this spring drift typically lasts about five days and is known to produce considerable ice jams as the breakup progresses northward.13