How can an ice jam cause flooding upstream?

An ice jam causes upstream flooding by acting as a physical barrier that obstructs the natural flow of a river.12

When the quantity of moving ice exceeds the river's transport capacity—often where the river slope transitions from steep to mild or where ice encounters an intact ice cover—the ice accumulates and builds to significant thicknesses. This blockage prevents the drainage of water from snowmelt or rain, causing the water level to rise behind the jam.12

As the water builds up, it can inundate surrounding areas upstream. This process can be dynamic; sometimes the water pressure causes the jam to break loose, only for it to reform further downstream, repeating the cycle of rising water levels and potential flooding. These events are particularly common during spring breakups when large quantities of ice are in motion.21