Tributaries flow into other bodies of water.
Watershed is the entire system of water flow and source.
Create a watershed in the dirt by making indentations for a lake, brook, and river. Demonstrate an idea of a watershed by pouring water into each of the features and watching the water flow from one elevation to another. Develop a definition of tributary and watershed with the students. Have the class work on a landscape to include mountain, river, lake, and as many other things as they would like. Carefully pour clean water into their water system for them.