One of the difficulties of modeling a flowing water system is the size of the system and the quickness of the flow. This often makes demonstrations hard to visualize as things happen so quickly. The model I designed uses large transparent plastic cups and clear tubing that connects them to easily demonstrate how a river can become contaminated by a toxic spill or dump and how the toxic material slowly works its way downstream creating devastation along the way. In time, the river will eventually run clean, but the damage takes much longer to disappear, and some damage may be permanent.
Begin by setting up a mock river system using six to ten separate cups. There is no limit to the number of cups you can use. The siphon tubes used to connect the cups are made of 6.4mm (inside diameter) clear tubing cut into 40cm lengths. You can use aquarium tubing and smaller plastic cups but a slower system will result. You will need one less tube than the number of cups you use. For a self emptying system, you can insert a smaller tube into the last cup that empties into a larger container.