If you throw a gum wrapper on the street, it could wind up in a river miles away.
Rainwater can wash the wrapper into the storm sewer. Then water in the storm sewer carries the wrapper through the underground world of pipes. Stormwater (and the gum wrapper) flow into a river miles away. The fish that live there and the people who swim and boat there aren’t too thrilled by the litter.
Any litter can end up in a river--even the “litter” your dog leaves on the ground. Pet poop pollutes.
Litter and leaves collect on a storm drain. If they move into the storm sewer, they could be carried to nearby rivers and lakes.