Supply Driven: Releasing What the River Actually Gives
Releases track the 3-year rolling average of natural flow. Highest median ending elevation — but a floor that can bite in consecutive dry years.
Supply Driven is the simplest of the DEIS alternatives and in some ways the most honest: the release rule is 65% of the 3-year rolling average natural inflow, bounded by a 4.7 MAF/yr floor and a 12 MAF/yr ceiling. In good years the lake releases more; in dry years it releases less. The approach mimics how unregulated rivers behave — supply-driven rather than demand-driven.
Projected Lake Powell elevation under the driest decade on record. Median line shows the most likely outcome; p10 line is the 10th-percentile worst case.
The scorecard
We ran this plan through our standard stress test: sampled inflows from the last ten years (the driest decade on record), 2,000 Monte Carlo iterations, 40 years forward. Same starting point as every other plan we evaluate. Same inflow sampling. Only the operating rule changes.
| Horizon | Median Ending | Worst-Case Floor | Grade |
|---|---|---|---|
| 10 years | 3625.1 ft | 3475.9 ft | B |
| 20 years | 3663.4 ft | 3474.9 ft | A |
| 40 years | 3670.2 ft | 3474 ft | A |
Strengths
- Highest 40-year median ending elevation of any plan we evaluated (3,669 ft). Under normal-to-good draws, the lake recovers strongly.
- Simple rule. Easier to explain, audit, and defend than multi-tier or dual-indicator schemes.
- Tracks the actual supply — so in dry decades it automatically tightens releases, while in wet decades it lets the system breathe.
Weaknesses
- 4.7 MAF/yr minimum release floor can force water out of Powell even in consecutive dry years where releases should arguably be lower. This drags the worst-case (p10) floor to 3,470 ft — below minimum power pool.
- The 4.7 MAF floor is a political commitment to Lower Basin minimum deliveries, not a hydrological necessity. It is the reason this plan grades B rather than A.
- Large year-to-year variability in releases. Downstream users have to manage around fluctuating deliveries, which creates its own costs.
Verdict
Supply Driven is a strong plan that is hamstrung by its floor. In median outcomes it beats every other plan; in worst-case outcomes the minimum-release constraint forces breakdowns. If the floor could be negotiated lower (say, to 4.0 MAF), this plan would be our top pick on simplicity grounds alone. As written, it's a B — a solid plan that narrowly misses the top spot because of one political compromise built into its definition.
See all plans compared in the head-to-head verdict, or read about why the real problem isn't drought — it's math.