Water resources systems are increasingly stressed by both climate and changing water demands. The uncertainty associated with these stressors greatly complicates risk assessments, especially because the relative and combined impacts of each stressor on water resources systems is often unknown. This study applies a bottom-up ex post scenario analysis approach (termed decision-scaling) to explore the spatiotemporal distribution of impacts from multiple stressors on a multi-objective transboundary river basin – the Apalachicola-Chattahoochee-Flint basin. The response of this large water resources system to variability and change in climate (specifically precipitation and temperature), as well as change in water demand (specifically municipal, industrial, and agriculture water demand), is examined using a novel “stress test” approach that simultaneously explores the relative and joint impacts of each stressor. The resulting system response is used to frame available projections of climate and water demand change in terms of risk to system performance. Additionally, an analysis of variance was conducted on the resulting ex post scenarios to attribute uncertainty in system response to uncertainty in projections of the various stressors. The findings of this analysis indicate that projected changes in mean precipitation are the dominant source of projected stress in the basin, while the impacts from temperature and demand vary in importance spatially. Early in the 55 year planning period, internal climate variability is the greatest source of uncertainty in system response, while by the end of the planning period uncertainty in projections of trends in precipitation dominates. Uncertainty in projections for other stressors also contributes to uncertainty in system response depending on spatial location. This study demonstrates the application of the decision-scaling methodology to a large, complicated, multi-objective basin with multiple stressors and yields important insights into water resources risk assessments for planning.