Salinity refers to the concentration of dissolved salts–most commonly sodium chloride (table salt NaCl) but also magnesium sulfate, potassium nitrate, and sodium bicarbonate. Salinity in the Colorado River increases as it flows downstream (Figure 1) due to accumulation and concentration of salts from both natural and man-made sources and causes. Natural sources (e.g. saline springs, concentration due to evaporation and transpiration, erosion and dissolution of sediments, etc.) account for approximately half of the salinity as measured at Hoover Dam. The remaining half of the salinity contributions are due to human activities and are split between out of basin exports (3%), agricultural irrigation (37%), reservoir evaporation and plant consumption (12%), and municipal and industrial uses (1%). In total, approximately 9 million tons of salts flow past the Hoover Dam annually.
Following protests in Mexico about the excessive salinity of the Colorado River water deliveries to Mexico and implicit violations of the 1944 US-Mexico Water Treaty, salinity control measures were established across the Colorado River Basin. The Colorado River Basin Salinity Control Act (1973) established salinity criteria at three monitoring locations along the River and required the implementation of salinity control programs. The monitoring locations and target salinity concentrations are: below Hoover Dam (723 mg/L), below Parker Dam (747 mg/L), and at Imperial Dam (879 mg/L).
The salinity control programs include salinity control units at Paradox Valley (CO), Grand Valley (CO), Las Vegas Wash (NV), and Crystal Geyser (UT) in addition to voluntary on-farm salinity control programs. Salinity control measures were estimated to control 1.22 million tons of salt annually in 2020 (about 10-12% of the salinity measured below Hoover Dam). Minute No. 242 mandates that the salinity of water deliveries to Mexico upstream of the Morelos Dam be no more than 115 ± 30 mg/L above average annual flow-weighted salinity of the Colorado River arriving at Imperial Dam.
Salinity is the only water quality issue the Bureau of Reclamation is obligated to monitor and address at the basin-scale; Reclamation spends approximately $35 million annually on salinity measures. The Bureau of Reclamation estimated in 2017 that high salinity in the Colorado River causes $300-$400 million in economic damages annually. Roughly half of these economic impacts occur in the agricultural sector, as many high-value crops are sensitive to high salinity. Other impacts include pipe corrosion and higher costs of water treatment. Rising salinity in the Salton Sea (CA) has deleterious effects on agriculture, human health, environmental health, and local economies.
Data and Tools
Salinity Control Monitoring Reports
Measured salinity at the established monitoring locations (below Hoover Dam, below Parker Dam, and at Imperial Dam) are reported in the appendices of the Salinity Control Monitoring Reports by the Colorado River Basin Salinity Control Forum.
Both historical and future salinity and salt loads have been estimated using a variety of models. NRCS USGS, and Reclamation have used the SLOAD, and SPARROW models to estimate salinity in the Colorado River.
SPARROW (SPAtially Referenced Regression On Watershed attributes) - USGS  SPARROW uses nonlinear regression equations to estimate the transport of analytes (such as salts) through river networks, including estimates of upper Colorado River Basin salinity
Substantial Declines in Salinity Observed Across the Upper Colorado River Basin During the 20th Century, 1929–2019 (Rumsey et al. 2021). Rumsey et al., 2021