Hi James,
The River Reborn was an early custom prototype story, so not all of its features have been incorporated into the Cascade builder. That layer toggle is one of the features that is currently not available.
It is possible for web developers to build that type of element if they download the Cascade source code, but it would be a significant effort. We hope to release a developer guide and some code samples in the future to help developers who are interested in customizing Cascade, but those materials are not ready at this time. We would also like to add that feature to the Cascade builder in the future, but we don't currently have a timeline on when it would be available.
Owen
Owen Evans
Lead Product Engineer | StoryMaps