Investigators attempting to demonstrate causal relationships in environmental systems face challenges associated with natural variability, multiple stressors, the difficulty of performing rigorous experiments, and the time and money required to undertake such studies. Rapid, transparent, and logical methods are needed to synthesize and evaluate evidence from multiple existing scientific research studies. Eco Evidence was developed for this purpose. It provides a synthesis method, online database, and analysis software for use in environmental causal assessments. Eco Evidence provides a definition of ‘acceptable’ and ‘sufficient evidence’ to establish causation. To illustrate its capabilities, we provide an example in which Eco Evidence informed a review of groundwater abstraction limits for environmental flow guidelines in the Australian Capital Territory. The synthesis of available evidence from peer-reviewed and unpublished sources was deemed sufficient and acceptable to inform a decision to keep groundwater abstraction limits <10% of long-term recharge. However, the definitions of acceptable and sufficient evidence might differ based upon the reasons for conducting an environmental causal assessment and on the perceived risk and consequences of making particular decisions.