Nature Note: Jam Sesh – Ice Jams on Lake Champlain’s Tributaries

Ice jams in rivers can quickly lead to serious flooding in a few different ways, including by pushing ice chunks on to river banks.

Flood watches were issued in parts of the Lake Champlain basin this March as warm temperatures thawed the frozen tributaries of the lake. Ice jams were reported on the Missisquoi and Mad Rivers in Vermont and the Ausable and Saranac Rivers in New York. With snowmelt raising water levels and large chunks of ice breaking up and floating downstream, ice jams become a real risk. What exactly is an ice jam, and what can we do to mitigate them?

According to the Ausable Freshwater Center, a non-profit organization dedicated to advancing science and stewardship of Adirondack waters, ice jams are the result of an imbalanced hydrological equation. They form when the amount of ice in the river exceeds a river channel’s transport capacity, or its ability to move things downstream. Both natural and human-caused phenomena can reduce transport capacity. River bends, shallow areas, low slopes, and solid ice that spans the channel are all natural conditions that can restrict flow. Human activity is often a significant factor in constricting the flow of ice, including dams (both functioning and derelict), bridges, eroded banks, and development that blocks a river from accessing its natural floodplains and adjacent wetlands. 

Like a traffic jam, ice jams are a backup of the mobile chunks of ice transported on a river. Such constriction heightens the risk of floods. As the backup accumulates, ice will fill the channel and dam water flow, causing upstream river levels to rise. Sometimes the jam itself can hop the banks onto surrounding areas, another flood risk. Ice jams can also release suddenly, sending a surge of water and ice downstream—yet another flood risk and a source of scouring, which alters the shape and character of a river.

Ice jams, like the floods they can cause, are a natural phenomenon that can happen more frequently and with greater consequences due to human development. Communities in floodplains are at greater risk during an ice jam, just as they are during summer floods. LCC continues to advocate for flood resilient policies that allow rivers to “breathe” when water levels are high through wetland protection.