These hanging dams eventually slow the current down to where it starts backing up, saturating the bright white frazil cover and loosening all the crystals which may have frozen together. The entire frazil surface gets raised up by the rising water and floats the cover above the sloping shores. You can tell when the river is backing up when you see greenish water amongst the frazil cover, pools form next to the shore, and you hear crackling from frozen parts of the pools.
When some of the hanging dams finally break, as they have to because the river flow is inexorable and forces its way downriver, the water level falls quickly and the whole cover of frazil lowers, settling onto the sloping shores on the edges of the river.
This process can happen a few times at the beginning of the winter, but eventually the cover stabilizes at a level slightly above the river, allowing the water to flow unhampered by hanging dams on its way to the ocean, and leaving frazil deposits 10 feet thick on shore after the main current washes away the cover over it. These 10-foot-high deposits can last into mid-April at the Warrensburg park next to Cronin's Golf Course.
Understand it all? I don’t either, but after 25 years of watching and studying I’m getting there.