Data & methodology
Every number on this site traces to a public agency source. Build stamp: 2026-07-28T00:01:27Z.
Reservoir data
All elevation, storage, inflow and release series come from the U.S. Bureau of Reclamation's Upper Colorado Basin hydrodata service (usbr.gov/uc/water/hydrodata), fetched nightly as published CSVs. USBR marks all values provisional and subject to revision; we re-fetch full series nightly, so upstream revisions flow through, and our raw-file archive preserves every version we have ever fetched. Lake Powell's daily record begins with Glen Canyon Dam's closure in 1963; Lake Mead's begins in 1935.
Derived numbers
- Records are computed from the full daily series and always state their definition (e.g. "full-series daily min" vs. "since-2000 daily min"). We never hand-enter a record. Powell's full-series minimum (3,394.5 ft, May 1964) occurred during initial filling, which is why "lowest since 2000" is reported separately.
- Date-in-history ranks ("Nth-lowest July 25 on record") compare today's value against every same-calendar-date value in the series.
- Water-year statistics use the October 1 – September 30 water year.
- Percent full is deliberately absent. It depends on which capacity table you divide by: USBR's original 1963 Lake Powell capacity (25.16 maf at elevation 3,700, per USBR's Glen Canyon Unit page) differs from today's sediment-adjusted live capacity. Until we publish versioned capacity tables with sources, we show acre-feet and let them speak.
Operating thresholds
Reference lines appear on charts only when the elevation is verified against a governing document. Currently verified for Lake Powell: full pool 3,700 ft and minimum power pool 3,490 ft (USBR Glen Canyon Unit). Pending verification: Powell's 3,525 ft drought-response target and 3,370 ft dead pool, and Lake Mead's shortage tiers, whose governing rules are being renegotiated for post-2026 operations. Thresholds are stored as versioned data with effective dates, because policy elevations change.
Weather
Observed temperatures come from NOAA GHCN-Daily stations: Page Municipal Airport (USW00003162, 1997–present, the only currently active station near the lake) and the discontinued Wahweap (USC00029114, 1961–2012), Page 4NNE coop (1957–2012) and Lees Ferry (1921–2012) stations for historical context. Forecasts are Open-Meteo model output, labeled as forecasts and fetched at build time. We do not mix modeled reanalysis into observed records.
Water temperature
There is no continuous official gauge for Lake Powell surface temperature — we verified this against the USGS instantaneous-values network (zero active water-temperature sites in the lake's bounding box). So the Lake Powell page shows a typical-surface-temperature climatology computed from the USGS Lake Powell limnology archive (1964–2022 data release, DOI 10.5066/P9ZIKVYW): monthly median and interquartile range of all top-1-meter temperature samples at the 28 monitoring stations within 15 km of Glen Canyon Dam or in Wahweap Bay, labeled as a historical average rather than a current reading. A satellite-estimated current surface temperature is a possible future addition and would be labeled as an estimate; we will never present modeled or estimated temperatures as measurements.
Planned
24-Month Study scenario projections (displayed as USBR's separate minimum / most probable / maximum scenarios, never as a statistical confidence band), CBRFC unregulated inflow outlooks, snowpack context, boat-ramp status (official NPS status kept separate from modeled usability), and a shoreline map built from USGS's Lake Powell extent polygons and topobathymetric DEM.