I saw street cones, signs, frisbees, and other things not of nature. Pollution is dangerous to the stream because all the objects in the stream are covered in its' own type of filth and could contaminate the stream. People pass by, and liter, they would just throw their wastes over bridges or roads that pss over the stream and of course therefore polluting the stream. And in some cases a person may not even intend to actually pollute the stream.
Some people toss their rubbish out of their automobiles and onto the asphalt.
But in time, the weather changes would eventually pick up and gather all of the wastes and blow them in the direction of the stream. In time it all would equal up to pollution of the stream.