Why Lake Mead and Lake Powell Are Shrinking: The Megadrought Explained

A 23-year drought, overallocation of water, and climate change are draining the Southwest's two largest reservoirs.

By Garret Merkley · Explainer · Jun 10, 2026

Branched from What Happens When Lake Mead and Lake Powell Reach Dead Pool

Quick take

Lake Mead and Lake Powell have lost more than half their capacity since 2000 due to a megadrought lasting over two decades.
The Colorado River was allocated more water than it actually carries—a problem masked for years by unusually wet decades.
Hotter temperatures now evaporate more water from the lakes and reduce snowmelt, accelerating the decline.
Unless water usage drops or the drought breaks, both lakes could approach unusable levels within years.

Lake Mead and Lake Powell are the two largest reservoirs in the United States, storing water that supplies 40 million people across seven states and Mexico. Since 2000, they have lost more than half their total capacity. This isn't a single drought—it's a megadrought, a 23-year stretch of below-average precipitation that has become the driest period in the Southwest in at least 1,200 years. The shrinking is driven by three overlapping forces: the Colorado River was promised to more people than it can actually serve, the region has gotten hotter, and the snow that normally feeds the river in spring is melting earlier and in smaller amounts.

The Colorado River Was Overallocated From the Start

In 1922, the Colorado River Compact divided the river's water among seven states and Mexico. The agreement was based on flow data from an unusually wet period, assuming the river would deliver 16.5 million acre-feet of water per year. The problem: the river's actual long-term average is closer to 12.5 million acre-feet. For decades, this mismatch didn't cause a crisis because Lakes Mead and Powell were filling up—they acted as a buffer, storing surplus water from wet years. But once they filled, that buffer disappeared. Now, the states are withdrawing more water than the river replenishes, and the lakes are the only cushion between supply and demand.

The Megadrought: More Than Just Low Rainfall

The current megadrought began around 2000 and is the driest 23-year period in the Southwest since at least 1200 CE, based on tree-ring data. Precipitation has been below average, but that's only part of the story. Rising temperatures—the region has warmed roughly 2 degrees Fahrenheit since 1980—have made the drought worse through two mechanisms. First, hotter air evaporates more water directly from the lakes themselves; Lake Mead alone loses about 600,000 acre-feet per year to evaporation, roughly the amount Las Vegas uses annually. Second, warmer temperatures cause snow in the Rocky Mountains to melt earlier in spring and more of it to evaporate before reaching the river. This shifts when water arrives and reduces total runoff, even in years with normal snowfall.

Why the Crisis Accelerated After 2000

For the 1980s and 1990s, the Colorado River ran wetter than average. Lakes Mead and Powell rose to near-full capacity, and the overallocation problem was hidden. In 2000, Lake Mead reached its highest level ever at 1,229 feet. But when the megadrought arrived and stuck around, the lakes began a steady decline. The problem compounded: as water levels drop, surface area shrinks, which should reduce evaporation. Instead, the effect is overwhelmed by rising temperatures and persistent low inflows. The lakes have become caught in a feedback loop—lower water levels expose lighter-colored rock that reflects more heat, potentially warming the surrounding air and intensifying local evaporation.

Why This Matters Now

Lake Mead and Lake Powell don't just hold water—they generate hydroelectric power, provide recreation, and regulate the river's flow to users downstream. As levels drop, power generation declines and costs rise. More urgently, both lakes are approaching levels where water intake becomes difficult or impossible. Lake Mead's critical threshold, called 'dead pool,' is around 1,050 feet; below that, gravity can't push water through the dam's outlets. Lake Powell's situation is similar. We're not there yet, but current projections suggest Lake Mead could approach dead pool within the next few years if conditions don't improve. The stakes are highest for agriculture, which uses about 80 percent of the Colorado River's water, but cities from Los Angeles to Phoenix to Las Vegas also depend on it.

The Numbers

Lake Mead capacity in 2000: 26.1 million acre-feet. Current (2024): ~8 million acre-feet.
Annual evaporation from Lake Mead: ~600,000 acre-feet.
Temperature increase in the Southwest since 1980: ~2°F.
Portion of Colorado River water used for agriculture: ~80%.

What Happens Next Depends on Three Factors

The trajectory of Lake Mead and Lake Powell depends on precipitation, temperature, and water demand. A return to average or above-average rainfall could stabilize or refill the lakes, but climate models suggest the Southwest is likely to remain drier and hotter than the 20th-century average. Water agencies have begun reducing allocations and encouraging conservation, but demand still exceeds supply. Some proposals involve reducing agricultural water use (which is often wasteful), importing water from elsewhere, or accepting that some regions will need less water. None of these solutions are simple or painless, but without change, the lakes will continue to fall.

Could the lakes refill if we got a really wet year?

A single wet year helps, but wouldn't reverse the long-term trend. The lakes need multiple years of above-average inflow to recover meaningfully. Even in wet years, rising temperatures now evaporate more water than before, so recovery is slower than it was historically.

Why don't we just build a new dam or reservoir?

The Colorado River is already heavily dammed. New reservoirs would face the same evaporation problem, especially in a hotter climate, and would cost billions. The real issue isn't storage—it's that demand exceeds supply.

What happens to Las Vegas and Phoenix if the lakes dry up?

Both cities have groundwater reserves and recycling programs, so they wouldn't run out of water immediately. But agriculture would be hit first and hardest, since it uses far more water and has fewer alternatives. Cities would face higher costs and restrictions.

Is this permanent, or will it get better?

Climate scientists expect the Southwest to remain drier than the 20th-century average for decades. The megadrought could ease, but the region is unlikely to return to the wet conditions that masked the overallocation problem for so long.

Why was the Colorado River Compact based on a wet period?

In 1922, scientists didn't have access to long-term climate data. They based their estimates on the 1900–1920 period, which happened to be unusually wet. Tree-ring studies done decades later revealed the mistake, but by then the water rights were locked in law.

Sources

U.S. Geological Survey: Colorado River Basin water supply and demand study (2012 update).
National Integrated Drought Information System: Southwest megadrought and climate change analysis.
Tree-ring research (Woodhouse et al., 2010): Demonstrates the megadrought is the driest 23-year period in 1,200+ years.
Bureau of Reclamation: Current Lake Mead and Lake Powell elevation and capacity data.