Scientists Trace the Disappearance of Water in the Colorado River Basin to Groundwater Losses in the Little Colorado and Little Snake River Systems

New research has pinpointed where significant amounts of water have gone missing in the overtaxed Colorado River Basin — not in surface reservoirs, but deep underground. A team led by scientists from the University of California, Los Angeles (UCLA) has found that declining groundwater storage in the Little Colorado River and Little Snake River sub-basins is a major contributor to the basin’s overall water deficit, revealing a hidden crisis beneath the landscape.

The Colorado River, which supplies water to 40 million people across seven U.S. States and two Mexican states, has been under severe stress due to prolonged drought, climate change, and over-allocation. While much attention has focused on declining levels in Lake Mead and Lake Powell, researchers say a significant portion of the water loss is occurring unseen — in aquifers that feed tributaries like the Little Colorado and Little Snake Rivers.

Study Reveals Groundwater Depletion in Key Tributaries

Using data from NASA’s Gravity Recovery and Climate Experiment (GRACE) satellites, combined with ground-based well measurements and hydrological modeling, the UCLA-led team analyzed changes in terrestrial water storage across the Colorado River Basin from 2002 to 2023. Their findings, published in the journal Geophysical Research Letters, show that while surface water losses are evident, the largest declines in total water storage are occurring in groundwater systems.

“We’ve known for years that the Colorado River Basin is losing water, but we couldn’t fully account for where it was going,” said Dr. Holly Ober, lead researcher at UCLA’s Institute of the Environment and Sustainability. “Our analysis shows that groundwater depletion in the Little Colorado River and Little Snake River basins explains a substantial portion of the missing water — water that isn’t showing up in reservoir levels but is critically important to basin-wide sustainability.”

The Little Colorado River, which flows through northern Arizona and feeds into the mainstem Colorado River near the Grand Canyon, and the Little Snake River, a tributary of the Yampa River in Wyoming and Colorado, are both critical sources of baseflow — the portion of river flow that comes from groundwater, especially during dry seasons.

Why Groundwater Matters in the Colorado River System

Groundwater acts as a natural buffer during droughts, slowly releasing water into rivers and streams when snowmelt and rainfall are scarce. In the arid Southwest, this subsurface storage is essential for maintaining river ecosystems, supporting agriculture, and supplying wells for rural communities.

However, decades of unregulated pumping, combined with reduced recharge due to higher temperatures and less snowpack, have led to chronic overdraft in many aquifers. Unlike surface water, which is governed by interstate compacts and visible in reservoirs, groundwater use is often poorly monitored and managed, making depletion difficult to detect without advanced sensing technologies.

The study estimates that groundwater losses in the Little Colorado and Little Snake River basins account for up to 30% of the total terrestrial water storage decline in the upper Colorado River Basin over the past two decades. This highlights a previously underappreciated dimension of the region’s water crisis.

Implications for Water Management and Policy

The findings have urgent implications for water managers tasked with implementing the Colorado River’s operating guidelines, which are currently being renegotiated ahead of the 2026 expiration of existing agreements. If groundwater depletion continues unchecked, it could undermine efforts to balance supply and demand, even if surface water allocations are reduced.

Experts recommend integrating groundwater monitoring into basin-wide water accounting systems and developing coordinated management strategies that treat surface and groundwater as interconnected resources. Some states, like Arizona and Colorado, have begun implementing groundwater management plans, but enforcement and data sharing remain inconsistent across jurisdictional boundaries.

“You can’t manage what you don’t measure,” said Ober. “Satellite data like GRACE gives us a powerful tool to spot the big picture, but we need on-the-ground investments in monitoring wells, metering, and regulation to protect these hidden water reserves.”

Climate Change Accelerates the Crisis

The study also found that the rate of groundwater depletion has increased since 2010, coinciding with intensifying drought conditions and rising temperatures. Warmer air increases evaporation and plant water use, reducing the amount of precipitation that recharges aquifers. At the same time, longer growing seasons and increased agricultural demand put additional stress on groundwater supplies.

Climate models project that the Southwest will become increasingly arid, making reliance on groundwater even more precarious. Without sustainable management, many aquifers may not recover, leading to permanent losses of storage capacity and long-term damage to river ecosystems.

Conclusion: A Hidden Crisis Demands Attention

While the shrinking reservoirs of Lake Mead and Lake Powell serve as visible symbols of the Colorado River’s decline, the real extent of the water loss may be happening out of sight — beneath the desert floor. The UCLA study underscores the need to confront groundwater depletion as a central challenge in securing the future of the Colorado River Basin.

As negotiations over the river’s future continue, policymakers, scientists, and water users must look beyond surface flows and address the underground systems that sustain them. Only by understanding and managing the full water cycle — both above and below ground — can the region hope to achieve resilience in an era of increasing scarcity.

Key Takeaways

• Groundwater depletion in the Little Colorado River and Little Snake River basins is a major contributor to water loss in the Colorado River Basin.
• Satellite data from NASA’s GRACE mission revealed that subsurface storage declines account for up to 30% of total terrestrial water losses in the upper basin.
• Groundwater provides critical baseflow to rivers, especially during droughts, and supports ecosystems, agriculture, and rural water supplies.
• Climate change is accelerating groundwater loss through reduced recharge and increased demand.
• Effective water management must integrate surface and groundwater resources and improve monitoring, regulation, and cross-state cooperation.

Frequently Asked Questions

Why hasn’t groundwater loss been a bigger part of the Colorado River conversation?

Groundwater is harder to measure and manage than surface water. It doesn’t show up in reservoir levels, and pumping is often unregulated or poorly tracked. New satellite technology is now making these losses visible.