river flow · United States
Colorado River at Lees Ferry, Arizona
Annual natural flow
Modelled demonstration series · not observed data
The Colorado Compact divided this river in 1922 using flow figures from an unusually wet stretch of years, and every allocation since has been built on that arithmetic. A chart like this is what it looks like when a legal entitlement and a physical river disagree. The river does not renegotiate.
75 years, modelled
Same reading, two normals
| Normal | Mean | σ | Pct | Return |
|---|---|---|---|---|
| 1951–1980 | 545 m³/s | −2.20σ | 1st | 1 in 72 years |
| 1991–2020 | 393 m³/s | −1.70σ | 4th | 1 in 23 years |
Today would be the lowest value in the entire 75-year modelled series. Past the series' standing low of 240 m³/s (1994). Nothing about the reading changes between rows — only the yardstick.
The season here
Flow at Lees Ferry is the upper basin's entire contribution — mostly Rocky Mountain snowmelt, delivered in a single spring pulse.
Where this series comes from
MODELLED SERIES — not observed. Parameters set from published Lees Ferry natural-flow climatology and the documented multi-decade decline in Colorado basin runoff; lognormal transform applied. The 75 annual values are synthesised by the seeded generator. Population is a rounded basin water-user estimate.
Every number on this page comes from a modelled series: 75 annual values synthesised by a seeded generator from published climatological parameters for Colorado River at Lees Ferry, Arizona. It shows how anomaly reasoning works. It is not a measurement and must not be cited as one.
How to read σ
σ counts standard deviations from this place's own normal. The sigma guide · return periods.
More river flow stations
Open Colorado River at Lees Ferry on the field →