Today Evelyn sent me a link to a wonderful YouTube video from Yosemite National Park that explains this seasonal event nicely - with some great footage. You can see that video here.
Here is what Evelyn once wrote to me about frazil ice:
Frazil ice first forms as tiny round crystals*after nucleating in some way that has long been a puzzle to scientists*throughout the river in cold weather. Turbulent super-cooled (slightly below 32*F) water tumbles the crystals around making them grow until they float at the surface in loose agglomerations. It looks like floating snow. If frazil touches something underwater, a deadfall or a rock, it sticks. It builds from the sides and from the middle of the river and can eventually form dams, such is the case at the Ice Meadows where the ice can grow to near glacial proportions. Cobbles and gravels at the bottom can be popped to the surface when the ice becomes buoyant enough. This process adds to the other river dynamics that are constantly moving sand, silt, gravel, and cobbles along the riverbed.
I don't know if Michigan gets any frazil ice...perhaps in the UP. It's worth finding some if you've never seen it.