Flood outlook, gauge by gauge.

From NOAA's National Water Prediction Service, which publishes the forecasts issued by the National Weather Service's thirteen River Forecast Centers. Both the flood categories and the stage thresholds behind them are NOAA's own — Driftwise re-serves them and does not recompute, reclassify, or reinterpret them. We harvest the national gauge network once an hour into our own store and serve this map from that copy, so NOAA receives one request per hour regardless of how many people are looking at the page.

They are the National Weather Service's flood categories, and each one is defined per gauge rather than nationally. Action stage is the level at which a forecaster or emergency manager needs to start preparing — it is below flood stage, not a flood. Minor flooding means minimal property damage but some public inconvenience. Moderate flooding means some structures and roads are inundated and evacuations may be needed. Major flooding means extensive inundation of structures and roads, with significant evacuations. Because the thresholds are set against what actually floods at each specific location, the same river can be at minor flooding at one gauge and no flooding at the next one downstream.

It is the highest level still ahead in that gauge's current forecast — a crest, not a reading. That is what makes this map forward-looking rather than a snapshot of now. It is a forecast, so it carries all the uncertainty any forecast does, and River Forecast Centers revise them as new rainfall and upstream observations arrive. Switch the panel to "observed" and you see the other half: what the water is actually doing at the gauge right now. The two frequently disagree, and that gap is usually the most useful thing on the page.

Because NOAA issues river forecasts for part of the network, and largely when there is something to forecast. On a quiet day only about one gauge in six carries a current forecast; the rest report observations only. Those gauges are drawn as hollow rings on this map — present, and explicitly not forecast. That distinction is deliberate and load-bearing: a gauge with no forecast is not a gauge forecast to stay dry, and colouring it as "no flooding" would invent reassurance out of missing data. If you need to know whether a specific river is expected to rise, the absence of a forecast is not an answer.

Hourly. The observed readings underneath move fastest — most gauges report every 15 to 60 minutes — while forecasts are typically issued once or twice a day outside an active event, and more often during one. An hourly refresh keeps pace with the observations and is never more than an hour behind a new forecast. The timestamp beside the map is the harvest time, not the time you loaded the page, and it is the honest bound on how current anything here is.

No. This is a map, not a warning service, and it can be up to an hour behind. Flood warnings, watches, and advisories are issued by your local National Weather Service office, and evacuation decisions are made by your local emergency management agency — those are the authorities, and they have information this page does not, including flash-flood risk on streams with no gauge at all. Use water.noaa.gov and weather.gov for anything you intend to act on.

Not usefully. This map is built from river gauges, and flash flooding is often exactly what happens where there is no gauge — small streams, urban drainage, burn scars, and areas that go from dry to dangerous in under an hour. A river gauge network is the wrong instrument for that, and a map showing no flooding at gauges says nothing about a flash-flood threat nearby. Flash flood warnings come from your local National Weather Service office.

Yes. The national gauge set covers all 50 states plus the District of Columbia and Puerto Rico, and each of those has its own page on this site. Coverage density varies a great deal — some states have several hundred forecast gauges and others a few dozen — which reflects where NOAA has gauges and forecast points, not where flooding happens.

They answer different questions from different agencies. The streamflow map ranks each USGS gauge's daily mean flow against its own historical record for that calendar day, so it tells you whether a river is unusually low or high for the season — it is a drought and low-water instrument as much as a high-water one, and it looks backward. This map carries NOAA's flood categories, is keyed to the stages that actually cause damage at each location, and looks forward. A river can be running well above normal on the streamflow map and nowhere near flood stage on this one.

Yes. NOAA flood categories are one of more than a dozen federal inputs Driftwise watches on every monitored river reach, alongside USGS gauge readings, the National Water Model, drought classification, EPA water-quality designations, wildfire proximity, and air quality. In the product, a forecast is resolved to the specific reaches and access points a crew is responsible for, so a reach forecast to cross into minor flooding routes recommended actions — access closures, gear retrieval, survey rescheduling — to the people who own them, rather than leaving someone to check a national map by hand.