Where does the data come from?
Every storage, inflow, release and elevation figure is a U.S. Bureau of Reclamation daily value. Storage comes from Reclamation's Hydrologic Database (HDB) web service; inflow, release and elevation come from Reclamation's Upper Colorado hydrodata service, which also carries Lake Mead, Lake Mohave and Lake Havasu. Both are public and need no key. The site fetches them once an hour and shows the date of the reading on every page. Nothing is typed in by hand except the capacity constants and the page text.
How is "percent full" calculated?
Percent full = current storage ÷ the capacity figure Reclamation itself divides by when it reports that reservoir, rounded to one decimal. Using Reclamation's own denominator means the site's percentages match Reclamation's reports. The figures in use are: Lake Mead: 26,120,000 acre-feet (USBR available capacity); Lake Powell: 23,314,000 acre-feet (live capacity, 2017 survey); Lake Mohave: 1,873,648 acre-feet (USBR available capacity); Lake Havasu: 570,253 acre-feet (USBR available capacity); Flaming Gorge: 3,788,900 acre-feet (total capacity); Navajo Reservoir: 1,696,000 acre-feet (live capacity); Blue Mesa Reservoir: 829,500 acre-feet (live capacity); Fontenelle Reservoir: 345,360 acre-feet (total capacity); Elephant Butte: 2,024,586 acre-feet (total capacity, 2007 survey); Morrow Point Reservoir: 117,190 acre-feet (total capacity). "Live capacity" excludes dead storage below the lowest outlet; Reclamation reports Lake Powell and the Upper Colorado reservoirs that way since the 2017 Lake Powell survey. Each reservoir page shows its basis under Capacity. A daily check compares the Lower Colorado figures with the ones Reclamation prints on its hourly data page and alerts us if they change.
How is the 7-day change calculated?
Percent full on the latest reading minus percent full on the reading seven days earlier (or the nearest earlier day if one is missing), in percentage points, rounded after subtracting. It is a change in stored water, not a rate or a forecast. Zero is shown as neutral, not as a fall.
How are the 1-year change and the 10-year high and low calculated?
From Reclamation's end-of-month storage series, converted to percent of capacity with the same denominator. The 1-year change compares today's reading with the end-of-month value twelve months before the latest complete month. The high and low are the extreme end-of-month values in the last ten years; if today's reading is outside that range, today is shown as the extreme and the month-end record is given underneath. Because these use month-end values, a brief peak or trough inside a month is not captured.
How are inflow, release and the water balance calculated?
For Lake Powell, Flaming Gorge, Navajo, Blue Mesa, Morrow Point and Fontenelle, inflow and release are Reclamation's published daily volumes in acre-feet, summed over the seven days ending on the latest storage reading. For Lake Mead, Lake Mohave and Lake Havasu, Reclamation publishes release but not inflow, so inflow is estimated as release plus the change in storage over the same days; that estimate leaves out evaporation and, at Havasu, the aqueduct diversions, so it is a floor and is labelled "estimated". "Change in storage" is always the difference between two daily storage readings, never derived from the flows.
What do the system status and the status colours mean?
They are this site's presentation scale, not official designations. The system figure is total storage divided by total capacity across the 9 Colorado River basin reservoirs tracked here (Elephant Butte is on the Rio Grande and is excluded): warning at or below 50%, critical at or below 30%. Each reservoir page states its own warning and critical marks. Official shortage conditions are declared by Reclamation each August from projected Lake Mead elevation, not from any percentage on this site.
Why are Lake Mead and Lake Powell shown separately rather than combined?
Reclamation and much reporting quote the two reservoirs' combined storage. The combined number hides which reservoir is falling and by how much, and the two are operated under different rules, so this site shows each on its own and gives the combined figure nowhere. The system total on the dashboard covers all tracked Colorado basin reservoirs, not just these two.
What is the elevation figure?
The water surface elevation in feet above sea level on the day of the reading, from Reclamation. Elevation is what the Colorado River shortage tiers are written in (for example Tier 1 below 1,075 feet at Lake Mead, measured on the January 1 projection), and what boat ramp and marina notices use. It is reported, not calculated.
How current is the data, and can it change?
Reclamation publishes one value per reservoir per day, usually for the previous day, and the site checks for new values every hour. Recent readings are provisional and Reclamation revises them, sometimes weeks later, so a figure you saw last week may differ from the archive. If a reservoir's latest reading is more than 6 days old, its page says so. If Reclamation's services cannot be reached at all, the site shows built-in approximate figures labelled "Estimated" everywhere they appear, retries within minutes, and the mailing-list digest is not sent until live data is back.
What checks catch bad data?
Every figure passes physical plausibility rules before it is shown, and the same rules run in a daily automated check that emails us if anything fails: a missing or stale reading, storage above capacity, a seven-day or month-to-month move too large to be real, implausible flows, or a capacity figure that no longer matches Reclamation's published one. The rules and the arithmetic are covered by automated tests, and the report is public at /api/health.
Does the site forecast future water levels?
No. Every number is arithmetic on Reclamation readings: the current value, the change over seven days and over a year, the ten-year high and low, and the seven-day water balance. Reclamation publishes official projections in its monthly 24-Month Study, linked from each reservoir page.