How we calculate these numbers

Sources and update frequency

Pages are built from USGS Water Services (daily means and instantaneous values), U.S. Bureau of Reclamation RISE (daily elevation items), and U.S. Army Corps of Engineers CWMS (hourly elevation, reduced to one value per local day). Each raw response is saved with the time it was fetched. The site is rebuilt from those files; when a source has no new data, no reading is filled in.

How this site is built

Data are fetched automatically on a schedule from the agency APIs listed above. Raw responses are saved with the fetch time. Pages are generated by a script without manual editing of numbers. Provisional data may be revised by the agency that collected them. Readings are never estimated or filled in.

Datum rules

Each lake uses one datum: the one tied to that agency’s pool elevation. USACE and Reclamation pools are read on the project datum, which for these reservoirs matches USGS parameter 62614 (NGVD29) rather than 62615 (NAVD88). Lake Powell is RISE item 508, checked against USGS 09379900 parameter 62614. If the two readings differ by more than 0.5 ft on the same day, the lake isn't published. At Powell, parameter 62615 reads 2.9 ft higher and is not compared with the 3,700 ft full pool.

Where USGS publishes only parameter 00062, the datum name has to appear in the saved station page or be confirmed by a same-day CWMS elevation within 0.5 ft. Gage-height conversions are not applied. Truman Lake shows how strict this is: USGS 06922440 is gage height, so the only accepted series is USACE CWMS HAST.Elev on NGVD29. If that series is not in the public CWMS response, Truman has no page.

Which lakes get a page

A page requires an official series, a reading newer than 7 days, at least a year between the first and last saved day, and a named datum. A lake that fails any of these gets no page at all, not a thin page with a noindex tag. After publication, a reading older than 48 hours is labeled “last reading” with its date. A reading older than 30 days is noindexed and dropped from the sitemap. This version has no state tables, so there is no separate "data unavailable" label.

Calculations

Below-pool is the latest elevation minus the official pool elevation, in feet, on the same datum. The same-day mean is the arithmetic mean of earlier years that have that month and day; the year count is the number of those years. Monthly high, low, and average use the daily values inside each calendar month. A 30-day change uses the saved day nearest the date 30 days earlier, within 3 days. Percent of capacity and percent of average appear only when an agency published them. Storage percent is never computed from elevation.

Great Lakes

Lake Michigan and Lake Erie are measured by NOAA CO-OPS on IGLD 1985. This build does not fetch NOAA, so those lakes are not published and there is no Great Lakes hub yet. Until a NOAA series is added, the header link points here.

Affiliation

BrimGauge is not affiliated with USGS, the Bureau of Reclamation, or the Army Corps of Engineers. Data are provisional, subject to revision by the agency that collected them.