Data & methodology
Every number on this site traces to a public agency source. Build stamp: 2026-10-30T15:53:51Z.
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.
Upper Colorado snowpack
The snowpack pages read daily snow water equivalent (WTEQ) and water-year precipitation (PREC) for 136 NRCS SNOTEL stations from the NRCS Air-Water Database REST API, together with NRCS's own 1991–2020 daily medians and averages for each station. The roster is every active SNOTEL station whose official hydrologic unit code begins with 14 (the Upper Colorado Region), frozen in August 2026 and shared with the winter outlook's basin index. Basins are the eight HUC-6 subregions of region 14, labeled with the common names used by the Bureau of Reclamation and the reference snowpack sites (the official WBD name appears on each basin page).
A basin's SWE is the mean of today's readings at its stations that reported within the last 3 days; its percent of normal is the sum of those readings over the sum of the same stations' normals for the date (the NRCS convention). The charts' normal curves average every roster station's normal, a fixed set. "Normal peak" is the highest point on the basin's 1991–2020 average curve; "percent of peak" compares today's SWE with it. The peak-by-water-year tables take the maximum of the basin-average daily SWE over the stations with data that year, so early years rest on fewer stations (the count is shown). Percentages are suppressed when the normal for the date is zero. Current-year values are provisional.
European rivers
Current readings come from PEGELONLINE (Germany's federal waterways agency, WSV, with Austrian, Swiss and Dutch partner gauges; license DL-DE Zero 2.0) and from EuRIS for the Danube in Austria, Slovakia, Hungary, Serbia, Bulgaria and Romania (information incorporated from EuRIS, eurisportal.eu). Both services publish about 30 days of history, so we archive hourly values nightly. Status colors use each gauge's own published marks: below the low-water reference (GlW in Germany, RNW in Austria), below mean low water (MNW), above mean high water (MHW), or above the highest navigable level (HSW). Gauges whose marks are not in our data show the reading, its 7-day change and its range in our archive, with no color judgment.
Same-season comparisons on the Rhine and the German Danube use WSV's archive of unchecked 15-minute readings back to January 1, 2000, reduced to daily averages and minimums. For a reading on a given date, "typical" is the middle 80% of daily averages within seven days either side of that date in every complete earlier year, and the season low is the lowest daily minimum in the same window. Official forecasts are shown as published by WSV or, via EuRIS, by the national services; HydroStatus does not adjust them.
Spring runoff outlook (HydroStatus, experimental)
The winter outlook page carries a HydroStatus forecast of April–July unregulated inflow to Lake Powell (USBR series 33), issued on the 1st, 8th, 15th and 22nd of November through March and on April 1. It gives the odds of below-, near- or above-normal runoff, where the categories are the thirds of 1991–2020 seasons: below 4,338 thousand acre-feet (KAF), near normal 4,338–7,778 KAF, above 7,778 KAF. It also gives a middle estimate with 50% and 80% ranges.
Inputs, all as of the day before each issue date: snow water equivalent and water-year precipitation at 57 NRCS SNOTEL stations in the Upper Colorado basin (every active station there with daily records back to at least October 1980), each expressed against its own history and averaged; the mean of Lake Powell's daily unregulated inflow since October 1; and a linear year term. Live SNOTEL values are provisional.
Model. A ridge regression on the logarithm of April–July inflow, with the penalty chosen by leave-one-out cross-validation, and a normal spread in log space set from the leave-one-out errors. Each issue date has its own fit. Forecasts use every completed season from 1981 on.
How it was tested. For each spring from 2001 to 2026 and each issue date, the model was rebuilt using only earlier seasons and asked to forecast that spring from that date's inputs, 26 forecasts per date. Scores compare it with even odds (33/33/33) and with four simple baselines: the last 30 years' frequencies, last year's flow, the long-term trend, and both together. The pass/fail checks were fixed before any result existed: beat the best baseline; bootstrap evidence that the edge is not luck (95% interval lower bound above −0.05 and at least 60% of resamples positive); acceptable odds in each category; ranges that caught the actual flow at least 35% (50% range) and 65% (80% range) of the time; and skill since 2000. Each forecast is labeled with its date's result: strong (passed every check), moderate (clear edge over even odds, failed another check, in practice the range-coverage check) or low (an edge that could be luck). The forecast is published at every date with that label. In testing, skill was low through mid-January, moderate or strong from late January, and strongest in March and on April 1. The ranges have run narrower than nominal, which each forecast states. An earlier version based on Pacific sea-surface temperatures failed the same test and was not published.
This is not an official forecast. The Colorado Basin River Forecast Center's water-supply forecast is the agency product.