Ice jam on the Connecticut River, January 19, 2018. Photo: NOAA.
River ice jams are a common spring hazard across many parts of the United States, particularly in northern regions and Alaska. They form when thawing river ice breaks apart and moves downstream, piling up at bends, narrows or around infrastructure such as bridges and dams. These blockages can restrict river flow and cause water levels to rapidly rise behind them. If an ice jam suddenly releases, it can send a surge of water downstream, flooding nearby communities with little warning.
When conditions favor the formation of a river ice jam, NOAA’s National Weather Service (NWS) may issue a flood watch, advisory or warning. These status notifications make continuous local monitoring essential for timely alerts that protect lives and property.
Earth-observing satellites are an efficient way to monitor long stretches of rivers for signs of ice movement and meltwater, complementing aerial surveys and ground-based observations. NOAA’s Joint Polar Satellite System (JPSS) satellites orbit the Earth from pole to pole multiple times each day, providing global coverage twice daily, and more frequently near the poles. This is especially valuable in remote regions such as Alaska, where surface measurements are often limited.
NOAA’s Enterprise Flood Mapping (EFM) uses observations from the Visible Infrared Imaging Radiometer Suite (VIIRS) onboard JPSS satellites to generate near real-time maps of river ice and flood extent, helping forecasters visualize conditions along major rivers. Because of their operational value, VIIRS Flood Mapping products are widely used by NWS River Forecast Centers, the Federal Emergency Management Agency (FEMA), the International Charter Space and Major Disasters, and other agencies to enhance situational awareness and support decision making before, during, and after river ice jam and flood events.
Snowmelt is another critical factor that influences river ice breakup and the potential for river flooding. Several VIIRS snow products help forecasters assess upstream conditions that can signal increased runoff and downstream impacts. The VIIRS Snowmelt RGB product, which combines red (R), green (G) and blue (B) VIIRS bands, distinguishes fresh, dry snow from older or melting snow, allowing early signs of melt to be identified before the runoff reaches rivers. In addition, the VIIRS Snow Cover product maps the extent of snow on the ground, supporting regional assessments of how much snowpack remains available to melt. These products provide valuable context for evaluating conditions that may increase the likelihood of ice jams and river flooding.
Left: NOAA-21 VIIRS Snowmelt RGB imagery from December 29, 2023, shows snow cover around the Missouri River in the central U.S. Dark blue indicates older or melting snow and bright blue shows fresher snow. This information helps forecasters assess snow type and age, supporting forecast verification and evaluation of runoff and flood potential. Credit: NOAA.
Right: The VIIRS Snow Cover product from January 23, 2025, shows snow cover across the Hudson River Valley. The Normalized Difference Snow Index (NDSI) ranges from 1 (no snow) to 100 (full cover), indicated by yellow to red shading. The Hudson River runs vertically through the center of the image, with surrounding areas exhibiting widespread moderate to high snow cover, reflected in NDSI values between 40 and 80. Source: NASA Worldview.
The LEO observations and products that support river ice, flood and snow monitoring are explored in greater detail in Feature 3: Mapping and Tracking River Ice Hazards From Low Earth Orbit of the 2025 LEO Science Digest. Released each January, the Digest highlights practical applications of satellite observations and shows how they enhance weather forecasting, improve environmental monitoring, support disaster response and benefit our nation’s economy.