The answer is 'Leaf'. River tributaries are the smallest parts of rivers, such as streams, that lead into a larger river. Headwater catchments contain tributaries, but tributaries may also occur all along the length of a river, and feed into the main stem of the river, depending on the land elevation. This is similar to a tree leaf. Leaves contain tiny veins, that join to form thicker veins, that then move into the branch and finally the main stem.
