RiverWare and the Colorado River Simulation System (CRSS) model have been central to long-term planning for Colorado River mainstem operations. For the Post-2026 Final Environmental Impact Statement (FEIS), the modeling framework may be familiar, but the questions being asked and the analytical challenges have evolved. Because future conditions, including hydrology, cannot be confidently predicted, each operating alternative is simulated using 400 streamflow traces that systematically test a broad range of future hydrologic conditions. CRSS and resource-specific model outputs are then analyzed using a Decision Making under Deep Uncertainty (DMDU) framework to answer complex questions such as: When are alternatives robust or vulnerable? What conditions drive those outcomes? What tradeoffs emerge under challenging hydrology? This presentation describes how CRSS and DMDU are used in the Post-2026 FEIS to compare alternatives and better understand performance given an uncertain future.