A plain-language explanation of the forecasting model, its data sources, and how well it performs.
The model answers a simple question: where will the water level be next month?
It uses a mass-balance approach. Think of Lake Powell as a giant bathtub. Water flows in (snowmelt, rain, tributaries), water flows out (releases to the Grand Canyon, evaporation), and the level changes accordingly. In equation form:
"Storage" is measured in million acre-feet (MAF) — the volume of water in the reservoir. But most people think in terms of the water surface elevation (feet above sea level), so the model converts between the two using a lookup table published by the Bureau of Reclamation (BOR).
All data is pulled from public federal sources. No proprietary data or API keys are required.
The BOR publishes a table mapping water elevation to storage volume. The model stores 20 points from this curve and interpolates linearly between them. Round-trip accuracy (elevation → storage → elevation) is better than 0.5 ft.
Key reference elevations: 3,700 ft is full pool (24.3 MAF), 3,490 ft is minimum power pool (5.0 MAF), and 3,370 ft is dead pool (1.7 MAF).
Each spring, snowmelt in the Rocky Mountains feeds the Colorado River and refills Lake Powell. The amount of snow on the ground — specifically the Snow Water Equivalent (SWE) on April 1 — is a strong predictor of how much water will flow in during spring and summer.
The model uses a linear regression of basin-average April 1 SWE (averaged across 10 stations) against the observed net storage gain from April through the summer peak (typically July). This is calibrated on 2020–2025 data, with R² = 0.899.
A key design choice: the regression predicts the net effect — inflow minus releases minus evaporation — not just raw inflow. This is more useful because it directly predicts what happens to the lake level, not an intermediate quantity you'd need to further adjust.
Outside the spring refill window (August through April), the model applies average monthly net storage changes derived from 2020–2025 historical data. During these months, releases exceed inflow, so the lake steadily declines:
| Month | Net Change (MAF) | Notes |
|---|---|---|
| August | −0.355 | Peak release month |
| September | −0.182 | |
| October | −0.073 | |
| November | −0.147 | |
| December | −0.225 | |
| January | −0.273 | |
| February | −0.221 | |
| March | −0.163 | |
| April | −0.041 | Transition to refill |
During spring refill (May–July), the SWE-derived net gain is distributed across three months: 35% in May, 45% in June, and 20% in July. This distribution reflects the typical timing of snowmelt runoff in the Upper Colorado Basin.
The interactive forecast allows users to adjust two parameters:
These controls let users explore "what-if" scenarios without modifying the underlying model.
The model was validated by hindcasting five known water years (2020–2024). For each year, the model starts from the actual January elevation and April 1 SWE reading, then forecasts forward. Results are compared against observed peak (summer high) and trough (winter low) elevations.
| Year | Start Elev | SWE | Pred. Peak | Actual Peak | Peak Err | Pred. Low | Actual Low | Low Err |
|---|---|---|---|---|---|---|---|---|
| 2020 | 3600.2 | 4412 | 3619.6 | 3610.6 | +9.0 | 3598.6 | 3559.4 | +39.2* |
| 2021 | 3566.2 | 3356 | 3566.2 | 3561.8 | +4.4 | 3531.3 | 3522.1 | +9.2 |
| 2022 | 3523.1 | 3662 | 3528.0 | 3539.5 | −11.5 | 3494.1 | 3519.5 | −25.4* |
| 2023 | 3521.6 | 5527 | 3580.6 | 3584.3 | −3.7 | 3553.9 | 3557.6 | −3.7 |
| 2024 | 3558.4 | 4445 | 3583.5 | 3586.8 | −3.3 | 3557.4 | 3557.3 | +0.1 |
* 2020 and 2022 had atypical release rates. WY2021 released 8.28 MAF vs. the current 7.48 MAF tier; WY2022 was drought-reduced to 7.07 MAF. The model is calibrated for the current Mid-Elevation Release Tier (7.48 MAF/year).
Bottom line: Under current operating conditions (2023–2024), forecast errors are under 4 feet for both peak and low predictions. When release rates deviate significantly from the calibration period, errors can reach 25–40 ft — which is expected, since the model assumes current policy will continue.
As of March 2026, Lake Powell sits at 3,529.4 ft with snowpack running at roughly 56% of the April 1 median. Under moderate assumptions, the model projects a low point around 3,488 ft by early 2027 — just below minimum power pool (3,490 ft).
This aligns directionally with the Bureau of Reclamation's own January 2026 24-Month Study, which projects 3,497 ft (most probable) to 3,479 ft (dry scenario) by end of 2026.