Widespread Drought-driven Declines in Streamflows and Water quality in the Upper Colorado River Basin (1998-2022)

The cascading effects of meteorological drought on water availability and quality in mountainous regions are poorly understood. The Upper Colorado River Basin (UCRB) is a vital water source for the American Southwest, primarily fed by snowpack in head- water catchments. Reduced precipitation and rising temperatures have caused droughts in the basin since 2000. We used a modular data-driven approach to study streamflow and water quality changes during severe droughts from 1998 to 2022 in the UCRB. Drought events in 2001-2002, 2012, 2018, and 2020-2021 resulted in annual median streamflows declines of approximately 15% (N=202 sites), water temperature increases by 9% (N=36 sites), and specific conductance increases by 5% (N=23 sites) relative to normal adjacent years. The greatest declines for all variables occurred in summer months, with notable reductions in peak flows. The impacts are transient, recovering to normal levels post-drought. Catchments with higher freshwater withdrawals, deciduous forest cover, mean air temperatures, bedrock residence times, and silt or clay content had greater declines in flows, while those with higher baseflow index, soil water retention and infiltration capacity exhibited greater resilience. These findings highlight the importance of catchment attributes and human activities in determining their vulnerability to droughts in mountainous regions