Watersheds
A watershed is an area that has combined rivers and lakes. Watersheds eventually drain into the ocean. A watershed divide is an imaginary line between the highest point and lowest point of the watershed, connecting them. Watersheds can easily be polluted when a person or thing dumps trash into the top of the river. Since water flows downward, the rest of the watershed is then polluted from the source at the top of the river. Surface runoff also plays a role in this because it can carry pollutants from people's homes into rivers and streams, where it then flows through the watershed. To help prevent this you can look at this link: http://www.simsbury-ct.gov/water-pollution-control/pages/ten-things-you-can-do-to-reduce-water-pollution
Point vs. Nonpoint Source Pollution
Point source pollution is where you can literally point to the pollution source.
Nonpoint source pollution is where you can see pollution but there isn't a specific place you can trace it to.