More than half of the world’s rivers crosses one or more dams before reaching the oceans by the start of the 21^st century, and the number might increase to 90% by 2030. Also, even though the construction of dams has slowed down in most developed nations after the first wave in the 20th century, there is a new wave of dam construction with the number of large hydroelectric dams projected to nearly double by 2030. Understanding the implications of reservoir expansion for meeting future needs of energy, water, and land and agricultural commodities, how they may alter trade, food prices, and energy investment requirements, and how they may alter the vulnerabilities of different regions to climate change remain open research questions. In this study, we use a long-term integrated human-Earth system model, GCAM, which incorporates interactions between population, economic growth, energy, land, and water resources in a consistent economic framework, to investigate these research questions. In this talk, I will focus on how to most effectively account for reservoir storage and expansion pathways in GCAM and the evolution of their management/uses over time. We explore multiple scenarios of reservoir expansions that are consistent with the Shared Socioeconomic Pathways (SSPs) and assess the role of reservoirs in alleviating future water scarcity conditions under projected climate conditions and evolving human systems, and their economic implications.