How to use flood inundation maps

Flood inundation maps show modeled areas that may be covered by water at selected stages or flows tied to a nearby streamgage or forecast point. For trip planning, begin with the gauge named by the map and make sure it is relevant to your chosen access site. Then select the stage nearest the current reading or expected level and look beyond the shoreline. The useful result is a picture of possible impacts to low ground, roads, ramp approaches, parking areas, trails, bridge approaches, and the route needed to leave after the trip.

These layers are especially useful because a gauge number alone does not tell you which practical features may be affected. A river can be below a named flood category yet still create trouble at a low ramp road, a narrow parking lot, or a local crossing. Conversely, a colored inundation area near an access site does not prove every paved surface is underwater. Read the map as a modeled flooding scenario, use the legend carefully, and identify the access features that matter to your own arrival, launch, parking, and return plan.

Use the latest observed gauge stage to understand conditions at the measurement site and its recent direction of travel. Check the observation time and the hydrograph trend rather than relying on a single number. A rising gauge can change the access picture between departure and return, while a falling gauge may still leave closures, debris, soft ground, or water across low spots. An observation is not a direct inspection of every road or ramp, particularly where tributaries, backwater, local runoff, or distance from the gauge affect conditions.

Use forecast stages to test the access plan against your whole time window: the drive in, time on the water, loading, and drive home. River forecasts are estimates that may be revised as rainfall, upstream flow, reservoir operations, and other inputs change. If available, review forecast uncertainty as well as the single official forecast line. When the forecast stage places modeled water on any necessary part of the access chain, treat that as a reason to delay, find a verified alternative, or skip—not as a prediction that conditions will be manageable.

Trace the trip as one continuous access chain instead of checking only the boat ramp. Include the main highway exit, local roads, final turn, low-water crossings, bridge approaches, driveway, ramp road, parking area, launch surface, shoreline path, and the route back out. Compare each link with the inundation layer at the current stage and at the highest credible stage during your outing. A ramp can appear clear while the approach road is flooded, or a road can be passable while parking or the turnaround area cannot be used safely.

Zooming in can help distinguish a nearby flooded field from water that may affect a required route, but do not assume map precision makes the outcome certain. Inundation boundaries and depth displays are modeled from available terrain and hydraulic information. Actual conditions can differ because channels shift, debris collects, drainage backs up, pavement is damaged, or water reaches access points by routes the map does not fully represent. Base imagery may also predate current construction or damage. Confirm road, ramp, park, marina, and public-land status with the responsible local operator or agency.

Choose GO only when the observed stage, expected stage, and official local information all support a clear access chain with a practical way out. The route should avoid known low spots, and no warning, closure, barricade, or restriction should conflict with the plan. Have an alternate route before loading the vehicle, allow extra time for a turnaround, and avoid treating a marginal opening as a commitment. GO means the available information supports travel and access; it does not mean water conditions will stay unchanged or that fishing will be productive.

Choose MAYBE when water is mapped near any needed road, ramp, parking area, trail, or exit route; when the gauge is rising; when rain could alter conditions; or when local access status cannot be confirmed. A MAYBE decision calls for waiting, moving to higher ground, choosing another verified site, or planning a no-launch alternative. Choose SKIP when modeled water covers a required feature, a forecast creates a credible risk during the trip window, or an official warning, evacuation direction, closure, or barricade applies. Do not depend on arriving early to beat worsening conditions.

Never drive or walk through floodwater, and never go around a barricade. Water can hide washed-out pavement, holes, debris, damaged ramp surfaces, electrical hazards, and strong current, while depth and road condition are difficult to judge from a vehicle. Darkness, rain, and glare can make the hazard harder to see. If water is across a road, path, ramp approach, or parking entrance, turn around and use a verified alternative. A map that appears to show shallow or limited flooding is never permission to test the route in person.

Recheck conditions before leaving home, before committing to the final access road, and before starting the trip back. Keep enough fuel, phone charge, daylight, and time to turn around without pressure. Tell someone where you expect to launch and when you plan to return, especially if routes pass near rivers, creeks, or low crossings. End the outing early if rainfall intensifies, water rises, visibility drops, or the exit plan becomes uncertain. Posted restrictions, emergency directions, and hazards you can safely observe always outrank an inundation layer, gauge reading, or forecast.

These references were consulted when this guide was generated. Check current official alerts and local rules before every trip.

• api.water.noaa.gov (api.water.noaa.gov)
• water.noaa.gov (water.noaa.gov)
• usgs.gov (usgs.gov)
• weather.gov (weather.gov)
• weather.gov (weather.gov)
• weather.gov (weather.gov)