There’s a mystery afoot in the upper Colorado River Basin: the amount of winter snowpack and the resulting spring and summer runoff haven’t added up in recent years. In 2021, for example, measurements of the snowpack looked close to normal (80% of average), but the streamflows in the spring and summer of that year were only 30% of average, much lower than what was expected. Where did that water go? In the megadrought-stricken Western US, understanding the causes of this mismatch is a question that the 40 million users of water sourced from the Colorado River urgently need answered.
The Colorado River is drying up due to a combination of chronic overuse of water resources and a historic drought. The dry period has lasted more than two decades, spurred by a warming climate primarily due to humans burning fossil fuels. These hotter and drier conditions in the western United States have drained the major constructed reservoirs on the river — Lake Powell on the Arizona-Utah border and Lake Mead on the Arizona-Nevada border — to levels not seen since these reservoirs were first filled decades ago.
While previous investigations of water resources in this region have largely focused on the effects of climate change, the current study also took into account the impact of plants’ complex responses to increased atmospheric carbon dioxide.
There are a number of factors that have contributed to the decline in the river’s water levels :
Climate change:
The Colorado River is a snow-fed river, and as the climate has warmed, the amount of snowpack in the Rocky Mountains has decreased. This has led to a decrease in the amount of water that flows into the river.
Increased water demand:
The population of the southwestern United States has been growing rapidly, and this has put a strain on water resources. The demand for water for agriculture, municipal use, and recreation has all increased, and this has led to a decrease in the amount of water that is available for the river.
Changes in land use:
The way that land is used in the Colorado River watershed has also changed over time. The conversion of forests and grasslands to cropland has increased the amount of water that is lost to evaporation and transpiration.
Temperatures have risen faster in this region than anywhere else in the lower 48 United States. Portions of the Basin have warmed more than double the global average. The average flow of the Colorado River has already declined nearly 20% since 2000, with half of that attributable to rising temperatures. Temperatures in the Basin are predicted to rise another 2–5 degrees Fahrenheit by 2050, which could reduce river flows by another 10 to 40%.
