How to Read a River Hydrograph: Observed vs. Forecast Water Levels
Read river stage, observation times and forecast crests without mistaking a gauge reading for water depth at your home. Includes a fictional worked example.
Start with the gauge, not the highest number
A hydrograph plots water information over time. It can show stage, streamflow or both, and can combine observations with forecasts. Before interpreting the curve, identify the river and gauge location. A graph for another gauge is not a substitute for the one you meant to check. This guide explains how to read the product; it is not a flood forecast for any real community.
Source: NWS: hydrograph education ↗
Separate what was observed from what is predicted
On a National Water Prediction Service hydrograph, read the labeled observed and forecast portions separately. The observed trace describes recorded water levels; the forecast trace describes expected levels. Check the time axis, time zone and forecast issue time. A future peak on the forecast line is not a measurement that has already happened. Compare the latest observation with its timestamp, not simply with the time you opened the website.
Stage is not the depth of the entire river
USGS defines stage as water height above a reference level at a gauge. Streamflow, or discharge, is a volume passing a location per unit of time. They answer different questions. A stage value does not by itself state water depth above the riverbed, across a road or inside a building. Keep the units and gauge reference with the number when taking notes.
Read thresholds at that location
The NWPS chart can display local flood-category thresholds alongside the trace. Compare the plotted level with thresholds for that same site. Do not transfer a threshold to another river or assume a shared category means identical local impacts. Open the gauge’s detailed information and impact descriptions for context; a single graph cannot answer every property-specific question.
A repeatable way to save the useful details
Our reading worksheet below uses invented values, not an official alert. For a real check, record the gauge name, latest observation and valid time, forecast issue time, anticipated peak and its time, and the source URL. Keep “observed” and “forecast” in separate notes. If a screenshot omits those labels, return to the original page before sharing it.
Use the table when the graph is hard to read
The NWPS quick-start guide describes a “View Tabular Data” option in the hydrograph menu. It separates observed and forecast values into tables and offers CSV downloads. That is useful for checking an exact timestamp rather than estimating a point by eye. For a real emergency, use current official warnings and local instructions; do not wait for a graph to resolve uncertainty.
Put it into practice
| Example field | Example value | How to read it |
|---|---|---|
| Gauge | Example River at Training Bridge | Use this same gauge throughout the comparison. |
| Latest observed stage | 8 ft at 10 AM | A recorded value for that time, not an afternoon forecast. |
| Forecast stage | 11 ft at 6 PM | An expected future value, not an observation. |
| Fictional flood stage | 10 ft | The example forecast exceeds this example threshold; this says nothing about a real address. |
| What is still unknown | Street and building impacts | Do not convert the example’s 1 ft above threshold into 1 ft of water at a home. |
Official sources
- NWS: hydrograph education ↗
- NWS: NWPS product and services guide ↗
- USGS: how streamflow is measured ↗
- NWS: NWPS quick-start guide and tabular data ↗
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