MISSOULA, Mont. — Rivers and streams across Montana see hundreds of ice jams per year that block water from flowing and potentially cause major flooding. Ice jams have kept the Jefferson River near flood stage for several days, but a flood advisory was finally lifted for the area on Friday.
The type of ice jams common for this time of year are called freeze-up ice jams. The ice prevents water from flowing down the river.
“That type of jam will start blocking water up and spreading it out in areas we normally wouldn’t see it,” National Weather Service hydrologist Ray Nickless said.
Freeze-up ice jams usually cause the biggest issues in western Montana.
“In western Montana we see ice jams start to form sooner. They form in December, sometimes even in November,” he said.
Ice jams start to break up around March, but break-up ice jams cause more issues in eastern Montana.
“It can break up and start moving downstream and block bridges and stuff like that,” said Nickless.
While both can cause major flooding, Nickless said western Montana does not currently have any major jams that are expected to cause problems.