Data & methodology
Every number on this site traces to a public agency source. Build stamp: 2026-10-16T13:57:40Z.
Reservoir data
Colorado River system 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.
California reservoirs come from the state's California Data Exchange Center (CDEC), fetched nightly: daily reservoir elevation (sensor 6), storage (15), inflow (76) and outflow (23). CDEC's older digitized records carry occasional single-day errors (for example a lone 3,997-ft reading in a series that tops out near 1,084 ft), so we filter isolated spikes on the slowly-varying elevation and storage series — a value is dropped when it departs sharply from its neighbours and immediately reverts. Inflow and outflow, which spike legitimately during floods, are left unfiltered. Most California daily records begin in the 1980s or 1990s.
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, and every threshold in our database carries its source. Policy thresholds are stored with effective dates, and charts draw each one only over the dates its rule is in force. When rules change we end-date the old entries instead of editing numbers, so the table below shows what changed. The 2007 Interim Guidelines and the 2019 Drought Response Operations Agreement governed through operating year 2026; the 2027–2028 Operating Guidelines, issued under Interior's August 2026 Record of Decision, govern from October 1, 2026 at Lake Powell (water year) and January 1, 2027 at Lake Mead (calendar year). Rows marked "table only" are real triggers left off the charts to keep them legible.
Lake Powell
| Elevation (ft) | Threshold | In force | Source |
|---|---|---|---|
| 3,700 | Full pool | physical / standing | USBR Glen Canyon Unit page |
| 3,665 | Consultation on releases above 8.23 maf (table only) | 2026-10-01 to 2028-09-30 | 2027-2028 Operating Guidelines Sec. 5.7.B.5 |
| 3,565 | Upper / mid operating range boundary (table only) | 2026-10-01 to 2028-09-30 | 2027-2028 Operating Guidelines Sec. 5.1.A-B |
| 3,540 | Low-elevation protection range below (table only) | 2026-10-01 to 2028-09-30 | 2027-2028 Operating Guidelines Sec. 5.1.C |
| 3,525 | Target elevation (drought response) | 2019-05-20 to 2026-09-30 | 2019 Drought Response Operations Agreement, p.2 |
| 3,510 | Operating floor | 2026-10-01 to 2028-09-30 | 2027-2028 Operating Guidelines Sec. 5.1 |
| 3,500 | Protected minimum | 2026-10-01 to 2028-09-30 | 2027-2028 Operating Guidelines Sec. 5.1 |
| 3,490 | Minimum power pool | physical / standing | USBR Glen Canyon Unit page |
| 3,370 | Dead pool | physical / standing | USGS GCMRC Lake Powell extent polygons |
Lake Mead
| Elevation (ft) | Threshold | In force | Source |
|---|---|---|---|
| 1,125 | Consultation on increasing apportionments (table only) | 2027-01-01 to 2028-12-31 | 2027-2028 Operating Guidelines Sec. 5.3.A.5 |
| 1,075 | Level 1 shortage trigger | 2007-12-13 to 2026-12-31 | 2007 Interim Guidelines ROD p.38 |
| 1,050 | Level 2 shortage trigger | 2007-12-13 to 2026-12-31 | 2007 Interim Guidelines ROD p.38 |
| 1,025 | Level 3 shortage trigger | 2007-12-13 to 2026-12-31 | 2007 Interim Guidelines ROD p.38 |
| 1,010 | Consultation trigger | 2027-01-01 to 2028-12-31 | 2027-2028 Operating Guidelines Sec. 5.3.A.4 |
| 1,000 | No ICS deliveries if below on January 1 (table only) | 2027-01-01 to 2028-12-31 | 2027-2028 Operating Guidelines Sec. 5.4.B.5-6 |
| 950 | Upper outlets only; no hydropower below | physical / standing | 2027-2028 Operating Guidelines Sec. 5.3 |
| 895 | Dead pool | physical / standing | USBR 2007 FEIS Appendix B |
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.
24-Month Study projections
Projection charts on the Lake Powell, Lake Mead and Colorado River pages come from Reclamation's monthly 24-Month Study, parsed from the published PDF tables (end-of-month elevation for each reservoir). Each run is shown as its own line with its issue month and its assumed Lake Powell water-year release, never as a statistical confidence band. When the 2027–2028 Operating Guidelines defer Powell's water-year release decision to April (§5.1.C.2 and Table 1, the case for water year 2027), USBR publishes two official Most Probable runs each month, one assuming a 7.0 maf and one a 6.0 maf release; we show both. The Probable Minimum and Probable Maximum runs, which USBR describes as bracketing about 80% of outcomes, are published only in some months, so their issue dates can lag the Most Probable runs. We show the newest issue of each.