The Colorado River and its network of tributaries supply water to seven U.S. states and Mexico, supporting drinking water, farming, and hydropower. A large share of that water begins as snow in the mountains, building up during winter before melting in spring.
Each year in early April, water managers analyze the snowpack to estimate how much water will flow into rivers in the months ahead. For decades, this method worked well.
But something changed around the year 2000.
Since then, forecasts based on snowpack have consistently overestimated how much water actually reaches the river. Even in years with decent snow, streamflow has come up short. This growing gap has puzzled scientists and water managers alike.
So where is the water going?
Warmer, Drier Springs Are Redirecting Snowmelt
New research from the University of Washington points to an unexpected culprit: a lack of spring rainfall.
The study found that warmer, drier spring conditions explain nearly 70% of the difference between predicted and actual river flows. With less rain falling in spring, plants rely more heavily on melting snow for water. That means less of that snowmelt makes its way into streams and rivers.
Dry conditions also tend to bring clearer skies. More sunshine boosts plant growth and increases evaporation from the soil, further reducing the amount of water that reaches the river.
The findings were published in Geophysical Research Letters.
“The period of time when we were wondering, ‘Oh no, where’s our water going?’ started around the same time when we saw this drop in spring precipitation — the beginning of the ‘Millennium drought,’ which started in 2000 and has been ongoing to the current day,” said lead author Daniel Hogan, a UW doctoral student in the civil and environmental engineering department. “We wanted to focus on the cascading consequences of this. Less springtime rain means you likely have fewer clouds. And if it’s going to be sunny, the plants are going to say, ‘Oh, I’m so happy. The snow just melted and I have a ton of water, so I’m going to grow like gangbusters.’ This research really centers the importance of studying the whole snow season, not just when the snowpack is the deepest.”
Not Evaporation Alone The Real Culprit Is Plants
To solve the mystery, researchers explored several possible explanations. One early idea was that more snow might be turning directly into water vapor, a process called sublimation.
But that turned out to account for only about 10% of the missing water.
Instead, the biggest factor is how ecosystems respond to changing spring conditions. Plants, from grasses to trees, act like natural pumps, pulling water from the soil and releasing it into the atmosphere.
“There are only so many possible culprits, so I started to compare things that might be important,” Hogan said. “And we saw that springtime changes are a lot more exaggerated than they are in other seasons. It’s this really dramatic shift where you’re going from feet of snowpack to wildflowers blooming over a very short amount of time, relatively speaking. And without spring rains, the plants — from wildflowers to trees — are like giant straws, all drawing on the snowpack.”
Recent research supports this idea. Studies now show that plants can continue drawing large amounts of water even during hot, dry conditions, tapping both snowmelt and groundwater and reducing the amount that reaches rivers.
Evidence Across the Upper Colorado River Basin
To better understand the process, the team analyzed 26 headwater basins across the Upper Colorado River Basin at different elevations. They combined decades of data, including streamflow and precipitation records going back to 1964, and modeled how much water vegetation would consume.
“We make an important assumption in the paper,” Hogan said. “We assume that the plants have an unlimited amount of water even with less-than-average precipitation, because they have access to snowmelt.”
The results were consistent across all locations. When spring rainfall was low, streamflow dropped.
Lower-elevation basins showed the largest declines. In these areas, snow melts earlier, giving plants more time to grow and absorb water before it can reach streams.
Why Forecasting Water Supply Is Getting Harder
The findings highlight a major challenge for water managers.
Current forecasting methods rely heavily on snowpack measurements taken in early April. But by that point, spring weather has not yet fully unfolded, and that is now proving to be a critical factor.
“April is when everybody wants to know how much water is in the snowpack each year,” Lundquist said. “But the problem with doing these calculations in April is that obviously spring hasn’t occurred yet. Now that we know spring rain is actually more important than rain any other times of the year, we’re going to have to get better at predicting what’s going to happen rainwise to make these April predictions more accurate.”
The research team is continuing to investigate what happens during spring, including whether patches of lingering snow act like small reservoirs that feed nearby plants over time.
A Growing Problem Across the Colorado River Basin
More recent data suggests this issue is part of a larger and worsening trend.
Since 1999, precipitation across the Colorado River Basin has declined by about 7%, and in some years only about half of the expected snowmelt actually reaches rivers and streams.
At the same time, rising temperatures are accelerating snowmelt and increasing water loss through evaporation and plant use. In extreme cases, snow is melting weeks earlier than normal, reducing the amount of water available later in the year.
Other studies also point to the growing importance of soil moisture and groundwater. Dry soils can absorb large amounts of snowmelt before it ever reaches streams, while groundwater losses across the basin have surged in recent decades.
Together, these factors are weakening the once reliable link between snowpack and river flow.
Rethinking How We Measure and Manage Water
The longer the Millennium drought continues, the more important these findings become. Water supply predictions, reservoir management, and long-term planning all depend on understanding how much water will actually reach the river.
This research suggests that focusing on winter snowpack alone is no longer enough.
Instead, scientists and water managers will need to better track spring rainfall, soil moisture, plant activity, and temperature trends to make accurate forecasts.
In other words, the answer to the Colorado River’s missing water mystery is not a single cause, but a chain reaction driven by a changing climate.
This research was funded by the National Science Foundation, the Sublimation of Snow Project and the Department of Energy Environmental System Science Division (the Seasonal Cycles Unravel Mysteries of Missing Mountain Water project).
