Environmental sustainability has become important considering limited natural resources and an ever increasing consumption. Sustainability assessment is a complex procedure which is used to quantify and measure sustainability of a system. It needs to be performed in an organized and coherent manner to avoid unintended negative consequences. A systems-level, overarching, integrated model coupled with underlying process models would provide structure to sustainability assessment of coupled systems, thereby making the procedure more effective. The first objective of the study is to illustrate such a systems-level, overarching, framework using a simple, idealized, hypothetical model of a watershed as an example. The essential elements of underlying process models are utilized in building a simple, System Dynamics (SD) model, which is integrated at the overarching level. Orientors and indicators are used to assess sustainability. Two additional scenarios are modeled in order to improve system sustainability: technological advancement by implementing a rainwater use policy in the existing watershed model; and human behavior modification by coupling a social model with the existing watershed model. This demonstrates the effectiveness of the System Dynamics modeling approach in integration of multiple, varied systems. The second and ultimate objective of this study is to propose a generic framework for the overarching, systems-level model, providing a definite structure but allowing others to define their own model elements and relationships. This would enable more effective sustainability assessment of coupled complex systems. / Master of Science