Substantial Declines In Salinity Observed Across The Upper Colorado River Basin During The 20th Century, 1929-2019

Salinity in the Colorado River Basin causes an estimated $300 to $400 million per year in economic damages in the United States. To inform and improve salinity-control efforts, this study quantifies long-term trends in salinity (dissolved solids) across the Upper Colorado River Basin (UCRB), including time periods prior to the construction of large dams and preceding the implementation of salinity-control projects. Weighted Regressions on Time, Discharge, and Season was used with data sets of dissolved-solids and specific-conductance measurements, collected as early as 1929, to evaluate long-term trends in dissolved-solids loads and concentrations in streams from 1929 to 2019 (n = 14). Results indicate that large, widespread, and sustained downward trends in dissolved-solids concentrations and loads occurred over the last 50-90 years. For 12 of the 14 stream sites with significant downward change, median declines of -38% (range of -14% to -57%) and -40% (range of -9 to -65%) were observed for flow-normalized concentration and load, respectively. Steepest rates of decline occurred from 1980 to 2000, coincident with the initiation of salinity-control efforts in the 1980s. However, there was a consistent slowing or reversing of downward trends after 2000 even though salinity-control efforts continued. Significant decreases in salinity occurred as early as the 1940s at some streams, indicating that, in addition to salinity-control projects, changes in land cover, land use, and/or climate substantially affect salinity transport in the UCRB. Observed dissolved-solids trends are likely the result of changes to watershed-related processes, not due to changes in the streamflow regime.