To read a river gauge, first confirm the river and monitoring location, then check the latest observation time, the water-level trend, and any forecast crest. Compare those readings with that gauge’s flood thresholds and local impact descriptions. A river stage is not the depth of water at your house, and a falling river does not establish that a road or building is safe.
If local officials tell you to evacuate, follow their instructions immediately. Do not wait for a graph to cross a particular line. Avoid walking or driving through floodwater. The National Weather Service’s flood safety guidance puts protective action ahead of watching the next reading.
Illustrative values, not a live forecast. Solid line: observations. Dashed line: forecast. Gauge height is not water depth at your home. Open the full-size diagram.
Find the right river gauge
Open the National Water Prediction Service, search for your river or location, and select a gauge on the map. Confirm the gauge name and map position. Open its full-information page for the hydrograph, flood impacts, gauge details and available historical crests. The official NWPS user guide explains these controls and how to view the readings as a table.
Bookmark the gauge page alongside your local emergency-management and road-closure sources. A useful question for local officials is: “Which forecast point and impact descriptions apply to this neighborhood or access road?” Choosing the closest map marker without checking the river reach can leave you monitoring the wrong location.
River stage, water depth and streamflow are different
River stage, also called gage height, describes the water surface relative to a reference level. Streamflow, or discharge, describes how much water passes a location in a period of time, often in cubic feet per second. The USGS explanation of streamflow measurement describes how stage measurements and field measurements are used to develop a relationship between the two.
A reading of 12 feet does not necessarily mean the river is 12 feet deep. It also does not mean a nearby home has 12 feet of water. The reference level is not simply the present river bottom. As the NWS explanation of negative river stages shows, even a zero or negative reading can occur while water remains in the channel.
Check the units and reference information before comparing numbers. Do not subtract a river stage from your home’s elevation to estimate flood depth. Without compatible elevation references and a valid local flood analysis, that arithmetic gives a misleading answer. Likewise, two different gauges showing 12 feet do not necessarily describe comparable conditions.
Read the hydrograph in this order
A hydrograph plots water level or flow against time. Use the legend to distinguish measured observations from predictions, and check the displayed time zone. The NWPS full-information page distinguishes its official hydrograph from National Water Model information. Keep the product label with any number you record. See the NWPS guide’s hydrograph and full-information sections.
A household checklist for interpreting a river gauge
| Check |
Record or ask |
| Gauge and river |
Save the exact gauge name and location, not just a screenshot of the curve. |
| Latest observation |
Write down the measured value and its timestamp. An open webpage is not proof of a new reading. |
| Recent trend |
Read several observations. Is the stage rising, holding roughly steady or falling? |
| Forecast |
Record the forecast issue time, predicted high point and expected timing. Label these as forecast values. |
| Thresholds and impacts |
Read the descriptions associated with this gauge. Look for named roads, low areas or structures relevant to your plans. |
| Local instructions |
Check current warnings, evacuation information and road closures before deciding whether to travel or return. |
This checklist organizes the information you need. It does not calculate your property’s flood risk or set a household evacuation threshold.
What flood stage and the categories mean
Flood stage is a location-specific gauge height associated with hazards to people, property or commerce. It need not be the same as the point where the river fills its banks. The NWS flooding FAQ explains that distinction.
Action stage signals a level at which the NWS or its partners may need to prepare for significant water conditions. Minor flooding can still threaten public safety even where property damage is limited. Moderate and major categories describe increasingly consequential flooding, including affected structures, roads and evacuations. Read the actual local impacts instead of treating “minor” as permission to cross a low road. These categories are described in NOAA’s hydrograph terminology.
A worked example: below flood stage but still rising
Hypothetical example, not a current forecast: A gauge’s latest observation is 14 feet at 8 a.m. Its listed flood stage is 16 feet. The current forecast shows a crest of 18 feet at 2 a.m. the next day.
The useful interpretation is that the observed river is currently 2 feet below that gauge’s flood stage, while the forecast crest is 2 feet above it. Neither difference tells you how much water will enter a basement. The overnight timing is a reason to review the latest local instructions and your household’s transportation and assistance arrangements now.
Record the forecast’s issue time as well as its crest time. Then check for updated information before acting. Do not turn the 2 a.m. forecast crest into a personal deadline to leave. Local officials may require action earlier, and the forecast can change.
Why the forecast crest can change
A forecast crest is a prediction of the high point at the forecast location. It can move up, down, earlier or later as forecasters receive new information. Observed rainfall, expected future rainfall and updated flow measurements can change river forecasts, as the NWS explains.
Some locations also provide probability graphics showing a range of possible river levels. These help communicate uncertainty and should not be read as a promise that the river cannot exceed a particular line. NWS hydrograph education explains how different weather scenarios produce different possible river levels. Keep that context when sharing a chart with family members.
What if readings are missing or look wrong?
Check the last observation time and any station notice. NOAA notes that cold weather and river ice can interfere with gauges or produce irregular readings. Some locations do not have a forecast displayed at all times. A missing forecast is not a prediction of no flooding. See NOAA’s hydrograph guidance.
If the gauge is unavailable, use current official weather alerts and your state and local emergency resources. Do not travel to a flooded bridge or riverbank to check the instrument yourself. Keep other warning channels available rather than relying on a single webpage.
Can I return home after the river falls?
A falling reading is useful information about that gauge, but it is not clearance to enter a flooded house. Floodwater can leave contamination, electrical hazards and structural damage, and roads may remain closed. The NWS advises waiting for authorities’ clearance before entering a flood-damaged building and respecting closure signs.
Once access is permitted, use our flood damage cleanup guide to organize the recovery work. Save dated observations and official messages in your incident documentation log. Label a gauge reading as a measurement at the named gauge, not a measurement inside your home.
For related decisions, read Flood Watch vs. Flood Warning and the storm and flood recovery hub.
Written by Water Damage Advisor. Sources checked November 14, 2026. The worked example is fictional. This article explains public river data and is not a live forecast or a property-specific flood assessment.