Where the flow is slowed by a flat reach an obstruction, such as a bridge or area of unbroken ice, the moving ice will pile and back up. Once the jam starts, it can thicken quickly, primarily by ice blocks turning under the stationary ice. A flood quickly follows. An ice jam can also freeze to a river bottom and banks with water flowing around or through channels in the jam. If another breakup occurs, the new flow of ice blocks will be stopped at the existing jam.
Flooding is inevitable.
In January 1994, thickly frozen rivers, aggravated by warm temperatures and a deluge of rain, released built-up ice jams. The cataclysm unleashed major damage on homes, autos, and disrupted traffic as ton floes pushed up from the Pomperaug River onto roads and private property. Ice in the Shetucket River in Norwich and the Baltic section of Sprague forced evacuations from the flooding and floes in early February 1994. The ice mantle on the Connecticut River threatened the thickly traveled bridges in Hartford and forced some creative answers. Engineers from the U.S. Army Corps of Engineers, State of Connecticut, and City of Hartford tried their best ideas to avert certain catastrophe.