This page tells you some ways pollution is caused.
It's actually made by a lot of stuff. Oil spills, smoking cigarettes on the ground, trash on the floor, burning coal and wood, it also can be caused by factories polluting the air by the smoke that it produces etc.. That's why animals can die by drinking the water if liter gets in the water.
These are fish killed by the polluted water, gross. The reason why these fish are dead is because the PH level in the water is to high. The PH level can go higher if people pollutes the water.