• We present a new methodology to design and evaluate policy mixes.
• The methodology addresses conflictive environmental management issues.
• It uses cross-impact balances (CIB), a qualitative network analysis.
• The methodology provides easy operationalizations of synergy and consistency.
• It allows assessing sustainability effects and robustness of policy mixes.
Environmental governance often requires fulfilling different objectives at the same time, as environmental but also economic and social objectives of different actors and on different scales. That means, it needs to address (potential) goal conflicts. This challenge of how to achieve policy coherence is currently discussed regarding the United Nations’ Sustainable Development Goals (SDG). It also touches on questions of policy design in general, on how to avoid contradiction and foster synergies between interdependent policy tools. This paper presents a new interdisciplinary methodology to design synergetic and sustainable policy mixes. It contributes a new approach for policy design processes addressing goal conflicts of sustainable development. The methodology uses cross-impact balance analysis (CIB), a qualitative form of systems analysis: A policy-interaction model is built by iterating between desk research and expert consultation regarding different objectives, policies to achieve them, and directed interactions between these policies. Analyzing the model allows identifying alternative policy mixes, which optimize different objectives at the same time, are free of internal contradictions and use synergies between policies. The methodology provides easy operationalizations for synergy and consistency and allows assessing sustainability performance and robustness of policy mixes. The methodology is suitable for both ex post evaluation of status quo and ex ante evaluation of alternative policy mixes. The methodology is applied to the water management in the Lurín river basin, Peru. We argue that the approach is transferable to further environmental issues and to SDG interactions on the policy level and supports policy coherence for sustainable development.