Most of the Colorado River’s water spends time in a reservoir before anyone uses it. A chain of them runs from Wyoming to the Mexican border, and two, Lake Powell and Lake Mead, hold the large majority. The rest are smaller Upper Basin savings accounts and Lower Basin delivery pools that together decide how much water reaches roughly 40 million people.
The short answer
The Colorado River system stores water in more than a dozen reservoirs, but two dominate. Lake Mead, behind Hoover Dam, and Lake Powell, behind Glen Canyon Dam, hold roughly 80% of the system’s storage between them. Behind those two sit the Upper Basin reservoirs, Flaming Gorge, Navajo, and Blue Mesa, plus the Lower Basin’s re-regulating pools, Lake Mohave and Lake Havasu. The major reservoirs can hold on the order of 60 million acre-feet at full pool. As of late June 2026, they held about a third of that.
The major Colorado River reservoirs at a glance
Seven reservoirs carry most of the system’s water. Here is each one, its dam, the river it sits on, and how much it holds at full pool. These are fixed design capacities from U.S. Bureau of Reclamation project data, not today’s levels. For how full Powell and Mead are right now, the callout above and the live comparison page carry the current numbers.
| Reservoir | Dam | River | Capacity at full pool | Basin role |
|---|---|---|---|---|
| Lake Mead | Hoover | Colorado | ~26 MAF | Lower Basin delivery hub |
| Lake Powell | Glen Canyon | Colorado | ~24.3 MAF | Upper Basin savings account |
| Flaming Gorge | Flaming Gorge | Green | ~3.8 MAF | Upper Basin storage |
| Lake Mohave | Davis | Colorado | ~1.8 MAF | Lower Basin re-regulation |
| Navajo | Navajo | San Juan | ~1.7 MAF | Upper Basin storage |
| Blue Mesa | Blue Mesa | Gunnison | ~0.9 MAF | Upper Basin (Aspinall Unit) |
| Lake Havasu | Parker | Colorado | ~0.6 MAF | Aqueduct intake |
A few smaller reservoirs round out the system, including Fontenelle on the Green River (~0.35 MAF) and Lake Pleasant, an off-river Central Arizona Project storage pool. But the seven above hold nearly all of it.
The two that hold the West’s water
Powell and Mead are not just the biggest reservoirs in the basin. They are the biggest in the country, and everything else is a rounding error by comparison. Mead holds about 26 million acre-feet at full pool and Powell about 24.3 million, so the two together store roughly 50 million acre-feet when full, close to 80% of the system’s entire capacity.
That concentration is why the whole Colorado River conversation usually comes down to these two lakes. When people ask how the river is doing, they are really asking about Powell and Mead. The live side-by-side page tracks how full each one is today, and which lake is in worse shape works through the surprising answer, since the downstream reservoir has recently held more water than the upstream one.
The Upper Basin’s savings accounts
Above Lake Powell, three more storage units hold the Upper Basin’s reserve. Flaming Gorge on the Green River, Navajo on the San Juan, and the Aspinall Unit on the Gunnison (whose largest reservoir is Blue Mesa) were built alongside Glen Canyon Dam under the 1956 Colorado River Storage Project. Reclamation reports that these four CRSP main storage units, Powell included, hold a combined 30.6 million acre-feet of live storage (USBR).
Those upstream reservoirs are more than scenery. Under the Drought Response Operations Agreement, Reclamation can release water from them to prop Lake Powell up when its inflow collapses. It has done exactly that. In 2021, managers moved 161,000 acre-feet out of Flaming Gorge and Blue Mesa to defend Powell’s 3,525-foot target elevation. Between April 2026 and April 2027, Reclamation planned to release 660,000 to 1 million acre-feet from Flaming Gorge alone. The mechanics of that fight to hold Powell up run through why Lake Powell is so low and the forecast page.
How water moves through the system
The reservoirs are plumbed together, and water generally runs downhill from the Upper Basin to the Lower. Glen Canyon Dam releases water from Powell to meet the Upper Basin’s delivery obligation to the Lower Basin and to generate power. That release lands in Lake Mead, which then delivers to Arizona, Nevada, California, and Mexico.
The 2007 Interim Guidelines set Powell’s annual release with a tier system tied to its elevation. When Powell is high, an equalization tier increases releases to balance storage with Mead; when it’s low, as it is now, releases are cut instead to hold Powell up. That coordinated-operations framework is the main reason the two reservoirs tend to rise and fall together. For water year 2026, Reclamation cut Glen Canyon Dam’s release to 6.0 million acre-feet, the lowest its rules allow (USBR).
Why the whole system is low
The system’s big storage reservoirs are drawn from the same shrinking river, and they have all fallen for the same two reasons. First, the drought: tree-ring reconstructions in Nature Climate Change found 2000–2021 was the driest 22-year period in the Southwest in about 1,200 years, made substantially worse by human-caused warming (Williams et al., 2022). Less snowpack and hotter soils mean less runoff into every reservoir.
Second, the Colorado River is over-allocated. The 1922 Compact divided more water than the river reliably delivers, and independent analyses put the structural deficit, the gap between what the basin uses and what the river provides, at roughly 1.2 to 1.5 million acre-feet a year. Lower supply meeting steady demand drained the system. As of late June 2026, total Colorado River storage sat near 33% of capacity, down from about 40% a year earlier (USBR). The dead pool tracker shows how much cushion Powell has left before its most serious thresholds.
Should Powell be drained into Mead?
As the reservoirs kept dropping in 2026, an old idea resurfaced: stop splitting the system’s water between two half-empty lakes and consolidate it in one. The Glen Canyon Institute has argued for years for a “Fill Mead First” approach, on the logic that two low reservoirs lose more water to evaporation and canyon-wall seepage than one fuller reservoir would. The system already sends Powell’s releases downstream into Mead every year, even the reduced ones, so it leans that way by design.
It remains a proposal, not policy, and it collides with a bigger unknown. The 2007 Interim Guidelines that coordinate Powell and Mead expire at the end of 2026, and the seven basin states are negotiating replacements through Reclamation’s Post-2026 process. Upper Basin states are pushing to protect Powell; Lower Basin states are pushing to protect Mead’s deliveries. Whichever way that lands will decide how the reservoirs share the shortage. For the honest read on whether any of them refill, see will Lake Powell fill back up.
Sources
- U.S. Bureau of Reclamation — Colorado River Storage Project (CRSP main storage units, 30.6 MAF combined live storage; Flaming Gorge, Navajo, Blue Mesa, Powell)
- U.S. Bureau of Reclamation — Hoover Dam & Lake Mead FAQs (Lake Mead capacity)
- U.S. Bureau of Reclamation — Glen Canyon Dam / Lake Powell operations (Lake Powell capacity; water year 2026 release cut to 6.0 MAF)
- U.S. Bureau of Reclamation — Drought Response Operations Agreement (target elevation 3,525 ft; 161,000 AF released from Flaming Gorge and Blue Mesa in 2021; 660,000–1,000,000 AF from Flaming Gorge for 2026–27)
- U.S. Bureau of Reclamation — Lower Colorado weekly hydrologic update (total system storage ~33% of capacity, June 2026)
- U.S. Bureau of Reclamation — Post-2026 Colorado River operations (2007 guidelines expiration and successor negotiations)
- Williams, Cook & Smerdon, “Rapid intensification of the emerging southwestern North American megadrought,” Nature Climate Change (2022)
- Glen Canyon Institute — Fill Mead First (proposal to consolidate storage in Lake Mead)