Nearly 40 million residents of California depend on streamflow that originates from the mountain snowpack. Over the last two decades, the Sierra Nevada, which provides most of the drinking water, agricultural water, and hydroelectric power to the state, has experienced more frequent and prolonged periods of drought than have ever been recorded in modern times. The comprehensive effects of these drought periods may not be understood for some time and there is substantial interest in anticipating the long-term effects of unprecedented drought upon water supply. Runoff efficiencies or the ratio of basin runoff to incoming precipitation show wide interannual variability and in some catchments were observed to be historically low for water year 2021. The change in runoff efficiency can magnify drought and linger for years. Here, we present quantitative results of how runoff efficiency patterns have changed over the last 20 years for more than a dozen large basins throughout the Sierra Nevada. Then, we investigate the primary contributing climatological factors that trigger declines in runoff efficiency. Results shed light on when to expect lower streamflow yields and equip water forecasters and managers with improved tools to respond to warming climate trends.