Floods
Annual Peak Flow and Record Crests Explained
By the Water Level Atlas team · Last reviewed Oct 21, 2026
A river's annual peak flow is the single highest instantaneous flow recorded at a gauge during one water year, which runs from October 1 to September 30. A crest is the highest water level a flood wave reaches as it passes a point. Peaks are recorded by the US Geological Survey (USGS) as flow; crests are usually given by the National Weather Service (NWS) as a water height in feet.
What is the annual peak flow series?
For each gauge, USGS keeps one number per year: the highest instantaneous streamflow during that water year, plus the date and, where available, the gauge height at the time. Line those numbers up and you have the annual peak flow series. At long-running gauges it can cover many decades.
This series is the raw material for flood statistics. USGS uses the annual peaks at a gauge to estimate how likely floods of different sizes are, including the so-called "100-year flood". See what is a 100-year flood.
What is a water year?
A water year is the 12 months from October 1 to September 30. It is named for the calendar year in which it ends. Example: water year 2026 runs from October 1, 2025 to September 30, 2026.
When you see "highest peak in water year 2026", a flood in November 2025 counts toward it.
Is the peak flow the same moment as the highest water?
Not always. USGS notes that the annual peak flow may not happen at the same time as the highest water level. Backwater (water held up by something downstream, such as another river, ice or a tide) can push the level up without more flow, and tides can do the same near the coast.
USGS also attaches codes to individual peaks. A few that matter to readers:
| Code meaning (USGS) | Why it matters |
|---|---|
| Discharge is an estimate | The value is less certain than a measured one |
| Affected by regulation or diversion | Dams or diversions upstream changed the flow |
| Historic peak | May come from outside the gauge's regular yearly record |
| Caused by snowmelt, hurricane, ice jam or debris dam breakup | Explains an unusual peak |
| Gage height from a different site and/or datum | The height cannot be compared directly with today's readings |
| Gage datum changed during the year | Same problem, within a single year |
What is a crest?
The NWS glossary defines a crest, in river terms, as the highest stage or level of a flood wave as it passes a point. When the NWS forecasts that a river will "crest at 24 ft on Friday", it means that is when and how high it expects the water to peak at that gauge.
NWS gauge pages on water.noaa.gov list historic crests for many forecast points: the highest water levels on record there, with dates. These are heights in feet on the gauge's own scale, which is why they can be compared with NOAA's flood stages. See flood stages explained.
Why can record crests be hard to compare with today?
Some records are very old, and the river and the gauge may have changed since. Three common reasons:
The datum changed. Gauge height is measured above a fixed reference level (the datum), not the river bed. If a gauge was moved, or its datum was changed, an old height may be on a different scale from today's. USGS flags peaks whose gauge height came from a different site or datum. See vertical datums.
A dam was built. A record flood from before an upstream dam was built happened in a different river. USGS notes that new river impoundments are one of the things that can change flood estimates, and it codes peaks affected by regulation.
The land changed. USGS lists urbanization, mining, agriculture and channelization among the changes that can affect a peak record. More paved surfaces, for example, can change how quickly rain reaches the river.
None of this makes old records wrong. It means they describe the river as it was. A crest from decades ago may not tell you exactly what the same flow would do today.
Can USGS peaks and NWS crests disagree?
They can. The USGS peak is the highest flow in a water year. An NWS crest is a water height. As noted above, the highest flow and the highest water level do not always happen at the same moment, and old heights may be on a different datum. Check what each number measures before comparing them.
Using this on Water Level Atlas
River pages show the highest annual peak from the USGS peak flow record and the historic crests NOAA publishes for that gauge, each with its date and source. Where NOAA publishes flood stages, you can compare today's gauge height with those crests on the same page. We show these records as published by USGS and NOAA and do not adjust them; see our methodology page.