The Colorado River watershed spans some 246,000 square miles. But most of its flow comes from a handful of snow-capped mountain ranges in southern Wyoming, western Colorado and northeastern Utah, according to water researcher and consultant Jeff Lukas.
The river passes through some of the most arid land in the country, so snow accumulation in these areas is critical. In most years, snowmelt is responsible for around 80% of the river’s water supply, Lukas said.
Many scientific studies have examined why there is less water flowing in the river. Almost all have found the fingerprints of man-made climate change.
“When you have more evaporation, you have less water left to go down the river,” Chris Milly, a US Geological Survey hydrologist who co-authored a study last year examining the river’s decline .
The first, and perhaps the most cited explanation, is the ongoing mega-drought, which began in 2000. As dry as it has been, a study published in the journal Science in 2020 found that this may be just the beginning. Past mega-droughts have lasted longer than today’s.
But the lack of snow and rain doesn’t fully explain what’s happening to the Colorado River. And droughts, after all, are temporary. Some scientists say the evidence shows the river’s dwindling supply is probably not so fleeting.
Brad Udall, a climatologist at Colorado State University, says the river’s fate is best summed up by another term: Aridification.
In general, aridification is a transition to a new climatic state dominated by water scarcity and driven by the effects of warmer temperatures. As of 2021, temperatures across the basin have increased by an average of 1.4 degrees Celsius (2.5 degrees Fahrenheit) over the last century.
As temperatures warm, the amount of precipitation that falls as snow decreases — and the snow that falls melts sooner, according to Milly.
How it works: The snowpack reflects much of the sun’s energy back into space. But because the snow melts earlier and leaves the ground exposed, more heat from the sun is absorbed by the ground. This leads to more evaporation, which means less runoff ends up in the river, Milly said.
Dry soils and thirsty plants also contribute to the problem. When soils are desiccated by high temperatures during the summer and fall months, it can lead to reductions in runoff that persist even a year later, Udall said.
Higher temperatures also mean the atmosphere is “thirstier” and able to hold more water. This increases evaporation losses from soils and water bodies.
This all points to an unpleasant conclusion, Udall said: Until humans can stop global warming, there’s likely to be even less water in the river in the future.
A 2020 study in the journal Science, co-authored by Milly, attempted to estimate the amount of less water. The authors found that further decreases in river flow are likely, regardless of the measures taken. But without any significant reduction in greenhouse gas emissions, the study found that the river flow could decrease by 31% by the middle of this century.