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.