No matter where you are, you're in a watershed. A watershed is the area of land where all of the water that is under it or drains off of it goes into the same place. Watersheds come in all shapes and sizes. They cross county, state, and national boundaries. Smaller watersheds are also a part of much larger watersheds. All the water that drains from Casper Mountain and into creeks and drainages, including the water that runs down your street and into the storm drain (stormwater), ends up in the North Platte River. And accordingly, Casper is in the North Platte Watershed. The appearance of our river has a lot to do with all the water that flows into it.
The water that flows into the North Platte River can carry a lot of things with it. The water carries pollutants that we can see: grass clippings, wrappers, cigarette butts, and cups. The water also carries pollutants we cannot see: bacteria from animal feces (fecal coliform), oil, dirt, and pesticides. While nature has its own system to take care of some of these pollutants, the altering of the landscape by streets, parking lots, construction, houses, and an increase in pets and other animals can strain this system and make it ineffective.