Pack the foam into the plastic bin so that it resembles a nature scene. One side of the bin will be higher than the other to represent mountains or other high elevation areas.

Press a small basin into the foam on the lower side to create a minter lake. Push down hard enough so that the foam stays in place and does not resume its original shape.

Wedge a small river running from the higher side of the basin down to the miniature lake by compressing the foam with your fingers. Alternatively, you may want to cut a small river out of the foam with a knife. Be careful to always cut away from your own body.

Cover the entire surface of your miniature watershed with aluminum foil. You may need to use more than one sheet. Glue another scrap of aluminum foil over any areas which tear while covering your model.

Glue your model buildings to the aluminum foil. Place the models in between the lake and the mountains. Verify that your buildings do not block the flow of the river.

Place bits of colored gelatin between the buildings and the river. Just put a little bit by each building. This represents pollution.

Spray water onto your model with the spray bottle. This represents rain. Watch how the water flows into the river and carries the gelatin into the lake.