Forecasted changes to climate and land use were used to model variations in the streamflow characteristics of Occoquan watershed and water quality in the Occoquan reservoir. The combination of these two driving forces has created four themes and an integrated complexly-linked watershed-reservoir model was used to run the simulations. Two emission scenarios from the fourth assessment report of the Intergovernmental Panel on Climate Change (IPCC), along with four General Circulation Models (GCMs) by using two statistical downscaling methods, were applied to drive the Hydrological Simulation Program - Fortran (HSPF) and CE-QUAL-W2 (W2) in two future time periods (2046-2065 and 2081-2100). Incorporation of these factors yielded 68 simulation models which were compared with historical streamflow and water quality data from the late 20th century. Climate change is projected to increase surface air temperature and precipitation depth in the study area in the future. Using climate change only, an increase in high and median flows and decrease in low flows are projected. Changes in flow characteristics are more pronounced when only future land use changes are considered, with increases in high, median and low flows. Under the joint examination of the driving forces, an amplifying effect on the high flows and median flows observed. In contrast, climate change is projected to dampen the extreme increases in the low flows created by the land use change. Surface water temperatures are projected to increase as a result of climate change in the Occoquan reservoir, while these changes are not very noticeable under the effect of land use change only. It is expected that higher water temperatures will promote decreased oxygen solubility and greater heterotrophy. Moreover, longer anoxic conditions are projected at the bottom of the reservoir. Results indicate that higher water temperature will increase the denitrifying capacity of the reservoir, especially during summer months, further reducing the nitrate concentration in the reservoir. / PHD / Water resources managers are facing a new set of challenges of developing strategies to address the regional impacts of climate change and land use change, especially in metropolitan areas. Simulating climate change and land use change scenarios can shed light on mitigation and adaption approaches for water resources management as well as future designs (for example, infrastructure, agriculture, irrigation, etc. among other sectors). The focus of this study is the Occoquan watershed with an area of 1530 km² (590 square miles) which includes the 1700-acre Occoquan reservoir that yields about 40% of the drinking water supply of near 2.0 million residents in northern Virginia. The Occoquan watershed located approximately 40 km to the southwest of Washington, D.C. and is situated in the Mid-Atlantic region of the United Sates with four distinct seasons and is part of a bigger watershed known as the Potomac River Watershed. The primary aim of this research is to provide an improved, quantitative understanding of the potential impacts of climate change and land use change on the Occoquan watershed. The findings of this research can benefit future water supply reliability and mitigation strategies in the study area considering this watershed’s essential role as a water supplier in northern Virginia.