Freshwater is one of the most valuable resources in Washington State. In recent decades, water supply has been affected due to climate change and population growth. Understanding changes in water supply and demand is crucial for ensuring an abundance of water for residential, economic, and industrial needs. The proposed research analyzes changes in the streamflow regime of the Cedar and Tolt Rivers which provide drinking water for the greater Seattle area. The main goal is to calculate the water budgets for the Cedar and Tolt watersheds and estimate how the inputs and outputs to these budgets change over the 21st century. An existing ensemble of streamflow projections for the Cedar and Tolt Rivers are used to analyze changes in water supply. The mean streamflow for each month is compared between a 30-year control period (water years 1971-2000) and a 30-year future period (water years 2031-2060). For each of these periods, I determine “optimistic” and “pessimistic” scenarios for the streamflow. For the “drought” month the highest streamflow value is considered as “optimistic”, and the lowest as “pessimistic” since the goal is to assess potential shortages. I use existing monthly demand values provided by Seattle Public Utilities and create different future scenarios, based on the predictions of population and employment growth. Supply and demand values are compared to evaluate (1) the potential for water shortage and (2) water management and conservation methods to satisfy the unmet demand. One potential water management method is the construction of a new reservoir. The results of the research are aimed at helping to inform society and water managers about the potential changes in the water system. Based on this information, they might be able to introduce changes in their future plans to accommodate the predicted needs.