Considerable research has focused on securing SCADA systems and the physical processes they control, but an effective framework for the real-time impact assessment of cyber attacks on SCADA systems is not yet available. This paper attempts to address the problem by proposing an innovative framework based on the mixed holistic reductionist methodology. The framework supports real-time impact assessments that take into account the interdependencies existing between critical infrastructures that are supervised and controlled by SCADA systems. Holistic and reductionist approaches are complementary approaches that support situation assessment and evaluations of the risk and consequences arising from infrastructure interdependencies. The application of the framework to a sample scenario on a realistic testbed demonstrates the effectiveness of the framework for risk and impact assessments.