Study area: We evaluated the Virgin, Verde, Salt, and Gila Rivers in the Lower Colorado River Basin. The watersheds have extents in Arizona, Utah, New Mexico, US and Sonora, MX. Study focus: We calculated trends in total dissolved solids (TDS) concentrations and fluxes with the Weighted Regressions on Time, Discharge and Season model. The modeling framework leverages daily streamflow and discrete water quality observations at specific monitoring sites. We evaluated trends for a common period (1976-2008) and the whole period of record at each monitoring site in terms of climate and anthropogenic controls. New hydrologic insights for the region: Three rivers had persistent TDS concentrations exceeding the EPA secondary drinking water standard. All were associated with a geologic source of TDS. We observed increases and decreases in TDS concentrations at our monitoring sites, contrasting with global freshwater salinization and declining TDS concentrations in the Upper Colorado River Basin (UCRB). We attributed concentration variability to wintertime hydroclimatic forcing, with secondary influences of human water use. Reservoirs may decrease TDS concentrations by 50%. Efficiency improvements in irrigation and mining water uses may decrease TDS concentrations, while municipal growth increases TDS concentrations. We observed TDS flux declines at most monitoring sites. We attributed up to 85% of the TDS flux trend to changes in streamflow arising from drought and groundwater use. This study informs salinity dynamics in arid and aridifying locations, including the UCRB.
