It's a rainy day, and rain is collecting on a houses roof.
The raindrop fell of the roof of the building. It's now flowing down the street, and is picking up oil and gasoline from cars.
The water continued flowing through the forest. It's picking up dirt and fertilizer that's on the ground.
The water flowed into a stream bringing the pollution, and polluting the stream
The water is now flowing into north landing river, then leading to flowing into the main water shed of the area.
The water flowed into back bay (the main watershed of the area), and is now polluting the water. Millions of water droplets go through this same cycle every storm. Water is getting more and more polluted.