Water for 40 Million: What USGS Is Learning in the Upper Colorado River Basin

The Colorado River Basin supplies water to more than 40 million people in the western United States and Mexico — including Denver, Las Vegas, Phoenix, Tucson, Los Angeles and San Diego — and irrigates about 16,000 square kilometers of farmland. Since 2000, the Southwest has been unusually dry, with low precipitation and warm temperatures. In 2021, the river's two great reservoirs, Lake Mead and Lake Powell, fell to their lowest levels on record, bringing unprecedented limits on water use.

As much as 90% of the basin's yearly runoff begins upstream of Lake Powell, in what USGS calls the Upper Basin. A USGS fact sheet describes three forces shaping that supply, and the agency's work on each with managers, cities, tribes and local, state and federal agencies.

The Upper Colorado River Basin: elevation, the perimeters of the 2020 wildfires, and USGS offices, precipitation gages and snow monitoring sites. Map: U.S. Geological Survey.

1. Snow and storage

In the high mountains of Colorado and Wyoming, most precipitation falls as snow, and the snowpack acts as a huge natural reservoir. Spring meltwater is caught in a network of large reservoirs for use later in the year. How much snow falls — and how air temperature, snow water content, melt rates and dust on snow shape runoff — can swing streamflow and water supply across the whole basin.

• In 2021, USGS set up a snow hydrology monitoring network in places older networks missed, from low and middle to high elevations, continuously measuring snowpack, soil moisture and weather and surveying snowpack by hand each year.
• Drones map snowpack by remote sensing, without ground crews.
• Lake Powell collects all the Upper Basin's streamflow. Starting in 2017, USGS surveyed its land and lakebed and updated its elevation–area–capacity relationships for the first time since 1986, showing how storage has changed as sediment has built up over the reservoir's life.

Snow science in the Upper Basin: (A) measuring snow water equivalent and temperature in a snow pit, (B) a site in the USGS snow hydrology monitoring network, and (C) an uncrewed aircraft system measuring snow depth and density by remote sensing. Photos: U.S. Geological Survey.

2. Wildfire and water

Warmer temperatures, earlier snowmelt and drought have made large, destructive wildfires more common in the West since the 1980s. Beyond the direct danger, fires wash sediment, ash, nutrients and carbon into streams, lakes and reservoirs, harming drinking water and aquatic life. Lost vegetation and changed soils — how well they absorb water, or repel it — alter snowpack and streamflow. Heavy rain before a burn recovers can trigger major erosion, and these fire-then-flood sequences are growing worse as Western storms intensify.

2020 was Colorado's worst wildfire season on record, with more than 600,000 acres burned. USGS studied the fires' effects on drinking water, native fish and snowpack, and set up a precipitation network to support early warnings of floods and debris flows along Interstate 70 in central Colorado.

The Grizzly Creek Fire burning on the slopes above the Colorado River in 2020. Photo: InciWeb.

3. Salt

Salinity — measured as total dissolved solids — degrades the river's water, costing an estimated $400 million a year in corroded infrastructure, water treatment and lost crop yields. The Upper Basin supplies most of the more than 7 million metric tons of dissolved solids that pass Hoover Dam each year, largely from soluble minerals in its rocks. Natural erosion and saline springs add some; irrigated farmland adds a great deal, as unlined canals and overwatered fields leach salts from the soil.

Lining canals and irrigating more efficiently can reduce it. USGS helps find salt sources, judge whether control measures work, track long-term trends, and show how changes in land use and management affect the salt load, informing mitigation at both basin and sub-basin scales.

(A) A saline spring in Colorado and (B) an irrigated field with salt residue. Photos: U.S. Geological Survey.

Why it matters

Understanding snow, fire and salt in the Upper Basin, USGS says, is key to meeting human, agricultural and ecological needs across a vast region that depends on this one river.

Sources

Based on "Applying U.S. Geological Survey science to understand effects to water supply in the Upper Colorado River Basin," U.S. Geological Survey Fact Sheet 2025–3036; a work of the United States government in the public domain. The map and photographs are reproduced from the fact sheet.