Know your watershed
Effective flood preparedness begins with understanding the watershed — the area of land that drains into the river or stream system affecting your location. A large watershed can accumulate runoff from rainfall hundreds of kilometers upstream, meaning that local clear skies do not mean flood risk is absent. River flood risk depends on what is happening upstream, not just overhead.
The USGS provides watershed delineation tools that allow anyone to identify the drainage area contributing to a specific river location. Understanding your watershed reveals which upstream areas to monitor for rainfall and which upstream gauge stations provide the best advance warning of rising water levels.
River gauges: your primary warning signal
USGS stream gauges measure real-time river stage (water level) and discharge (flow volume) at thousands of locations across the United States. These gauges are the foundation of flood monitoring. Each gauge has defined flood stages — action stage, minor flood stage, moderate flood stage, and major flood stage — based on the specific impacts that occur at each water level at that location.
The NWS Advanced Hydrologic Prediction Service converts gauge data and weather forecasts into river stage predictions — showing the expected water level over the next several days. During rising water conditions, these forecasts update multiple times daily and provide the most specific information available about when and to what level flooding will occur at gauged locations.
National Weather Service flood products
The NWS issues a hierarchy of flood products. Flood watches indicate that flooding is possible based on weather forecasts. Flood warnings indicate that flooding is occurring or imminent based on observed conditions. Flash flood warnings indicate that rapid-onset flooding is occurring or imminent, typically from intense rainfall over a short period.
River flood warnings include specific predicted crest heights and timing. Flash flood warnings are more urgent and less specific — they indicate that life-threatening flooding is occurring now in the warned area. Understanding the difference between a river flood warning (which may give hours to days of lead time) and a flash flood warning (which requires immediate action) is critical for appropriate response.
Satellite flood mapping for broader context
Satellite-based flood mapping provides a broader spatial view than gauge networks. The Dartmouth Flood Observatory and GDACS flood products use satellite imagery to estimate flood extent over large areas, particularly useful for understanding the full scope of basin-scale flooding events. These products complement gauge data by showing which areas between gauge stations are inundated.
PlanetSentry displays flood events from EONET and GDACS that incorporate satellite-based assessments. These events provide regional and global context — showing how a local flood fits into broader patterns of river basin flooding, monsoon-driven events, or multi-region storm impacts. The combination of local gauge data and satellite-based regional assessment gives the most complete flood monitoring picture.
Personal preparedness actions
Beyond monitoring data, flood preparedness includes practical actions. Know your property's flood risk (FEMA flood maps show designated flood zones). Have a communication plan with family members. Keep important documents in waterproof storage. Know your evacuation routes and which roads may be impassable during flooding. Never drive through flooded roadways — it is the leading cause of flood-related deaths.
For communities in flood-prone areas, maintaining relationships with local emergency management agencies is valuable. These agencies often provide location-specific flood preparedness guidance, sandbag distribution during rising water events, and evacuation coordination when conditions warrant. The best monitoring data in the world is only useful if it connects to protective action.