Not the easiest challenge. Knew I would need a spatial function to create intersection object. Realized when they did not overlap i would need a buffer. Took a second to think of the configuration and transformation to buffer both polygons that were meant to be a section of the river. Settled on recordID and transpose.
Still needed some hardcoding of looking at the map and noting the pairs of states in a text input. Ollie Clarke's solution including a distance tool and filtering to those in an easterly direction is a very clever way of going about it in workflow.