The study also shows the majority of salinity in the Upper Colorado River Basin comes from groundwater that discharges into streams, also known as baseflow. This occurs as snowmelt and precipitation infiltrate into the ground and interact with sedimentary rocks, which causes salts to dissolve and salinity to increase in groundwater.

Salinity has a significant impact on water users in the Colorado River Basin, affecting agricultural, municipal and industrial sectors, and causing almost $300 million per year in economic damages in the United States.

Findings show that as much as 89 percent of salinity in the upper Colorado River comes from baseflow. The study estimated salinity loads in baseflow at 69 stream sites throughout the basin to better understand where salinity originates and how it is transported through the watershed. The study also examined salinity trends in baseflow from 1986-2011 to learn how conditions have changed over time. USGS scientists developed models and incorporated long-term data from sites throughout the Upper Colorado River Basin to provide estimates of how much salinity moves from groundwater to streams.

“Understanding how salinity moves through the Colorado River basin is critical for resource managers in helping them develop effective mitigation strategies,” said Christine Rumsey, a USGS scientist and the lead author of the study.

Declines in baseflow salinity loads occurred in 63 percent of streams studied between 1986-2011. This decline suggests that salinity mitigation projects may be reducing loads. Other possible causes for the decreased salinity transport include climate and landscape changes. Notably, the pace and extent of decreases in baseflow salinity declined during the 2000s. The average rate of decreases during the 2000s was only half of the average rate of decreases in the 1990s.

“While this is a great first step toward understanding how salinity has changed over time, more studies are needed to better understand why salinity loads are declining in the basin, and why, at many sites, the rate of decline was weaker in more recent years,” said Rumsey.

Salinity occurs naturally in water due to the weathering and dissolution of minerals in soil and rock. The same process occurs in areas with irrigated agriculture, which produces about double the salinity yield compared to areas without irrigated agriculture. Other factors known to affect salinity loads in streams include geology, land cover, land-use practices and precipitation.

Funding for this study was provided by the Bureau of Reclamation Colorado River Basin Salinity Control Program. In 1974, Congress enacted the Colorado River Basin Salinity Control Act, which directed the Secretary of the Interior to proceed with a program to enhance and protect the quality of water available in the Colorado River for use in the U. S. and Republic of Mexico. The Colorado River Basin Salinity Control Program implements and manages projects to reduce salinity loads, investing millions of dollars per year in irrigation upgrades, canal projects and other mitigation strategies.

The new study was published in the journal Hydrologic Processes.

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