A lake level is only useful if it is correct and you know when it was taken. These are the rules we follow on every page.
1. Where the data comes from
- U.S. Geological Survey (USGS): the Water Data API, with readings usually every 15 minutes to 1 hour. Real-time USGS data is provisional and subject to revision.
- U.S. Army Corps of Engineers (USACE): the CWMS Data API, for lakes run by its districts. Some of its series republish data from USGS or from local authorities; the page says so when that is known.
- U.S. Bureau of Reclamation (USBR): the RISE database, with one value per day for western reservoirs.
2. How often it updates
We download the data and rebuild every page every 3 hours. Each page shows the time of the reading, in the lake’s local time, and never the time the page was built. Daily series have a date but no time, so we show only the date.
A reading counts as current if it is less than 6 hours old (sub-daily series) or less than 48 hours past the end of its day (daily series). If it is older, the page says “Last reported” with a visible notice and the lake leaves the rankings of today.
3. Datums
An elevation is a height above a reference, called a datum: NGVD 1929, NAVD 1988 or a local one. The same lake can read several feet apart depending on the datum, so we never mix datums in one comparison or chart, and each page names the datum it uses. “Station datum” means the gauge’s own reference, which the source does not tie to a national one. Gage height is not an elevation: we show it labeled as such and never compare it with full pool.
4. Full pool
We only show full pool from an official source, cited on the page. For most lakes it is a reference level published by the U.S. Army Corps of Engineers (top of conservation, top of normal or top of gross pool). Many of these targets change with the season: we show the value for the current date and say that it is seasonal. A lake without an official figure shows no comparison, and we do not estimate one. We do not compare dry flood-control reservoirs, which hold no permanent lake.
5. What we check before publishing
- Values that are negative (except for lakes below sea level), above 12,000 ft, or that change by more than 10 ft in under 24 hours are discarded and logged, never corrected.
- Readings flagged by USGS for faulty equipment, ice, maintenance or a changed datum are discarded.
- Forecasts are never used as readings, and values dated in the future are rejected.
- Official sources sometimes publish impossible values with a good quality flag. A USACE reading outside the range between the dam’s streambed and its top is discarded. When several series of one lake disagree by more than 10 ft, a value that has clearly lost digits is dropped, then the majority wins, then USGS; if that cannot settle it, we show no value rather than a wrong one. Lakes with a single series cannot be cross-checked this way.
- If too many lakes have no recent data, a major source is down, or Lake Mead or Lake Powell have no data in 72 hours, nothing is published and the last good version stays online.
6. Trends, rankings and history
Trends compare the latest reading with the daily value 1 and 7 days earlier, on the same datum. We store one value per day per lake. If we have no value for that day, we show no trend. Rankings only include lakes with an official full pool and a current reading.
7. Limits
LakeLevelHQ is not an official site. The data is for information only: it is not for navigation or flood safety decisions. Check the official source linked on each lake page. Have you seen an error? Tell us and we will review it.