A weight-of-evidence (WOE) framework was developed to evaluate potential effects on the aquatic ecosystem of Wabamun Lake (Alberta, Canada) associated with the release of Bunker “C” oil after a train derailment. The wide variety of stakeholders and interested regulatory agencies made it necessary to develop a consistent and transparent approach to assessing ecological effects on multiple ecosystem components within the lake with the use of a large number of lines of evidence (LOEs). Consequently, a scalable WOE framework was necessary to integrate the findings of 38 different LOEs. A priori and a posteriori weighting factors were applied to each individual LOE, and a combination of numeric and nonnumeric rating systems was used to integrate LOEs into an overall WOE conclusion for 5 different ecosystem components. We provide guidance regarding the development of a WOE framework and emphasize techniques that enhance the application of best professional judgement during the WOE process.