Your model will look as though you've filled the basin to the rim or higher with lumpy mashed potatoes, then shoved half the potatoes to one end into a lumpy mountain. Use the marker to draw the place where you believe the main rivers will flow. Count the number of small watersheds that drain into the main river you drew with the marker. Notice how all the water flows towards one end of the tub. All the area drained by a river system is that river's watershed.
Ridges divide individual watersheds.
Explain that each small stream is formed by its own watershed. As streams join together, their combined watersheds and stream flow form larger watersheds and rivers. Switch tasks within your group and make another watershed model. This time, rearrange the rocks in the tub to try to create a model that illustrates two different watersheds. Both watersheds should drain into a lake at the lower end of the container.