Rivers & gauges
Provisional vs Approved USGS Data: What It Means
By the Water Level Atlas team · Last reviewed Oct 21, 2026
Provisional data are the live readings a USGS gauge sends in before anyone at USGS has checked them. Approved data are the same records after USGS staff have reviewed them against field measurements, corrected them where needed and cleared them for publication. Provisional values can change, sometimes a lot, which is why our pages label them.
What does "provisional" mean?
USGS says its real-time data are preliminary or provisional and subject to revision. They have not received final approval from USGS. USGS releases them anyway because timely information is useful, for example during a rising river.
Provisional data have only had automatic checks. USGS says improbable values sent by satellite or other telemetry are screened automatically. A person has not yet compared them with field visits.
Why can provisional values change?
A gauge measures water height every 15 minutes. Flow is then calculated from that height using a rating curve built from field measurements. Several things can make the first numbers wrong:
- Ice or debris. Ice, log jams or other debris can hold water back, raising the height without more flow. USGS lists backwater from ice or debris as a cause of revisions.
- Weeds and sediment. Algae and other aquatic growth, and moving sand and gravel, change how much water passes at a given height.
- Equipment problems. Sensors and recorders can malfunction.
- A changed river channel. When a field measurement shows the river no longer fits the rating curve, USGS applies a correction it calls a shift. Those corrections may arrive after the live number was posted. See gauge height vs streamflow.
USGS warns that subsequent review based on field inspections and measurements may result in substantial revisions.
How does data get approved?
USGS hydrographers visit gauges regularly to measure flow by hand and check the sensors. Back at the office, USGS staff review each station's record. They compare it with those field measurements and apply corrections where needed. Each station's record stays provisional until it is reviewed, approved and published.
| Provisional | Approved | |
|---|---|---|
| When | Minutes after the reading | Later, after review |
| Checked by | Automatic screening | USGS staff, against field measurements |
| Can it change? | Yes, sometimes substantially | Rarely; it is the official record |
| Best for | Seeing what is happening now | Records, statistics, long-term comparisons |
USGS says data are usually published within 6 months after the end of the water year on September 30. Because the water year starts on October 1, a reading taken early in the water year can stay provisional for well over a year.
What do notes like "Ice" or "Equipment malfunction" mean?
Along with its approval status, a USGS value can carry a qualifier: a short note about that observation. USGS gives examples such as a sensor that may have been affected by ice, or a value that was estimated. Other notes flag readings affected by equipment malfunction.
In practice:
- Ice-affected: ice at the gauge can hold water back and change the link between height and flow, so the value may be revised later.
- Equipment malfunction: the sensor or recorder had a problem, so values may be missing or wrong until fixed.
- Estimated: USGS filled the value using other information rather than the normal calculation.
When a value carries a note like this, treat it with more caution, especially if it looks unusual.
Should I trust provisional numbers?
They are the most up-to-date information available. But USGS says provisional data may be inaccurate, and asks users to consider that before using them for decisions involving safety or significant money. For flooding, follow the National Weather Service and local authorities, not a single provisional reading.
Provisional data also affect comparisons. A "record" in provisional data may not survive review. See annual peak flow and record crests.
Using this on Water Level Atlas
Our river pages show the latest USGS reading with the time it was measured and label it "provisional". Readings that USGS flags as affected by ice, equipment malfunction or similar problems are left out rather than shown as measured values, and estimated values are stored as estimates. Records and "normal for the date" ranges are based on USGS statistics; see normal for the date and our methodology page.