The major reason why the fish in the rivers would die is because of the excessive organisms in the water, so the bacteria get to multiply, thereby consuming most of the dissolved oxygen. As the organisms are black, when the river patrol officers see the water turn black, they can go upstream and find out from where the water is discharged. For example, municipal wastewater not properly processed, and effluent from paper factories, petrochemical factories and livestock farms contain large amounts of organisms might lead to the reduction in dissolved oxygen in water.