Answer :

wawa3hunna
because they are inconsiderate and don't really care about animals and or plant't or anything that lives in rivers they think about themselves but mostly people don't think about things before they do them they just act they don't really think they are hurting anything but they're hurting gods creation that's why people pollute.
People pollute rivers for many reason. 

1, For agriculture- People plant their crops and the fertilizers sometimes flush in to rivers and the river pollutes, killing millions of organisms.

2. Industries- People throw machinery tools into rivers and also fumes from the industries kills animals.

People pollute rivers because they just want to pollute the environment / they don't do environmental sanitation. They don't care if animals and people get sick. They are also risking their life.