Real-time river flooding
across America.
Live USGS streamgauge readings, FEMA flood zones, NWS flood watches and warnings, and historical context — one map, refreshed throughout the day. Built for flood researchers, emergency managers, and outdoor recreationists.
What causes river flooding
Flooding is rarely a single-cause event — multiple factors usually compound. The most common drivers across the U.S.
Heavy rainfall
Persistent rain saturates soils and overwhelms drainage networks. Tropical systems and atmospheric rivers are the worst culprits.
Rapid snowmelt
Spring melt pulses can deliver months of accumulated water in days — especially when warm rain falls on existing snowpack.
Ice jams
Breakup ice can block channels, forcing water to back up and inundate upstream banks. Common on northern rivers in early spring.
Storm surge
Coastal hurricanes push seawater inland. Surge combined with rainfall is the deadliest flood scenario in U.S. history.
Reservoir releases
Controlled dam releases can dramatically increase downstream flow. USACE and USBR publish release schedules, but conditions change fast.
Burn scars
Wildfire-stripped slopes can't absorb rainfall — even modest storms produce dangerous flash floods on burned watersheds for years afterward.
Flood preparedness checklist
Floodwaters rise faster than most people expect. The basics that save lives.
Flood map & river monitoring FAQ
What does "percent of normal" mean?
The current flow at a gauge compared to its seasonal average for this date. 100% means flow is right at the historical norm. 200%+ means twice the typical flow — a strong indicator of flood conditions on small-to-medium rivers.
What's the difference between a flood watch and a flood warning?
Watch: conditions are favorable for flooding within the next 12–48 hours. Warning: flooding is happening or imminent. Both come from the National Weather Service. Snoflo overlays both as toggleable layers on the map above.
How often does Snoflo's data refresh?
USGS streamgauge readings update every 15 minutes; we re-pull every hour. NWS warning polygons update as the NWS issues them — usually within 5 minutes. FEMA flood zones are static (the National Flood Hazard Layer is updated quarterly).
What is the FEMA flood zone layer?
FEMA's National Flood Hazard Layer (NFHL) shows the 1%-annual-chance ("100-year") and 0.2%-annual-chance ("500-year") floodplains. These are based on long-term hydrologic modeling, not current conditions. Useful for property risk; not a real-time signal.
Can I get an alert when my local river floods?
Yes. Save any USGS gauge as a favorite in the Snoflo iOS app, set a threshold (e.g. "alert me at 20 ft stage"), and you'll get a push the moment it crosses. Free with a Snoflo account.
Is Snoflo a substitute for official warnings?
No. Snoflo is informational. For life-safety decisions always follow guidance from local emergency management, the NWS, and law enforcement.