The River trait is shared between two Regions that share a border. Fleets with the Riverine Trait can cross the border between two Regions that share a River border, allowing them to move into and operate within inland Regions. They always use the lower of the two Movement Cost numbers when they do so. In order to enter the river a Fleet must be in the Region where the River’s mouth is located, represented by a River icon in the ocean area of that Region on the map.
When a Force that lacks either the Fleet or Marines traits crosses a border with the River Trait, the Movement Cost of that crossing is doubled unless there is a Bridge on that border as well.