Large Decreases in Upper Colorado River Salinity Since 1929

Salinity is the concentration of dissolved salt in water. High salinity levels in the Colorado River Basin cause an estimated $300-400 million per year in economic damages across U.S. agricultural, municipal and industrial sectors, as well as negatively impact municipal and agricultural users in Mexico. Reducing high salinity levels can benefit crop production, and decrease water treatment costs and damage to water supply infrastructure.

Findings indicate that large, widespread and sustained downward trends in salinity occurred over the last 50 to 90 years, with salinity levels decreasing by as much as 50% at some locations. The timing and amount of salinity reductions suggest that changes in land cover, land use and climate, in addition to salinity-control measures, substantially affect how dissolved salts find their way into streams that feed the basin.

'Identifying the causes of dropping salinity levels will be important for water managers in the basin so they can anticipate future changes in salinity and optimize salinity-control practices going forward,' said Christine Rumsey, USGS scientist and lead author of the study.

Results show the steepest rates of decline in salinity occurred from 1980 to 2000, coincident with the initiation of salinity-control efforts in the 1980s. However, there has been a consistent slowing of downward trends after 2000 even though salinity-control efforts continued. Significant decreases in salinity occurred as early as the 1940s in some streams, indicating that, in addition to salinity-control projects, other watershed factors are important drivers of salinity change.

'Having access to almost a century's worth of salinity data provides greater insight to the water-quality changes that occurred prior to the implementation of salinity-control projects,' said Don Barnett, Executive Director of the Colorado River Basin Salinity Control Forum. 'These findings are key in helping us understand the processes that cause and reduce salinity and assist us in our goal of protecting water quality in the Colorado River.'

Salinity occurs naturally in water due to weathering and the breaking down of minerals in soils and rock. The same process occurs in areas with irrigated agriculture, when irrigation water flows through soils and dissolves salts which eventually travel into streams. Irrigated areas contribute significantly more to stream salinity compared to areas without irrigated agriculture. Other factors known to affect salinity include geology, land cover, land-use practices, precipitation and climate.

'These findings indicate the issue of salinity in the Colorado River Basin is very complex,' said Rumsey. 'Further work is needed to better understand the roles that climate change, land-use, reservoirs, population dynamics and irrigation practices play in salinity issues, which impact the economic well-being of the West and are important to U.S. relations with Mexico.'

Funding for this study was provided by the Colorado River Basin Salinity Control Program, the Bureau of Reclamation and the Bureau of Land Management. 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 programs to reduce salinity loads, investing millions of dollars per year in irrigation upgrades, canal projects and other mitigation strategies.

The USGS is the primary scientific agency for collecting data on water quality and flow in the nation's rivers, with more than 13,500 real-time stream, lake and reservoir, precipitation and groundwater data stations across the country. The USGS also conducts analyses of these data to evaluate the status and trends of water-quality conditions.

The new study was published in the journal Water Resources Research.

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