The Nation Report: What are ice jams, and how do they form? The Nation Posted: Dec 5, 2019 / 04:35 PM MST / Updated: Dec 5, 2019 / 04:37 PM MST GRAND JUNCTION, Colo. — Chief Meteorologist Kris Nation explains ‘ice jams’ on Thursday’s Nation Report. Ice jams are a build-up of ice in rivers during winter. When temperatures warm up, ice jams come pouring down, often causing flood warnings.