River pollution in most of the places is caused by discharges from municipal sewage treatment plants, runoff from urban streets, septic tanks, landfills and from agriculture. Forestry and industry also contribute to river pollution to a lesser extent. Most of the seriously polluted stretches are affected by sewage discharges but these are mostly in small rivers that can be remediated through the upgrading of the sewage treatment facilities. Oxygen is a gas that does not dissolve very well in water and, at 5oC, saturation with oxygen is reached at 13 parts per million in rivers and lakes. All aquatic life depends on oxygen for survival and so anything that reduces the oxygen level can have a serious effect on the flora and fauna of the river. When organic waste is discharged to a river the bacterial breakdown of that material in the river uses up dissolved oxygen. Similarly green plant material absorbs oxygen from the water during the hours of darkness. If too much oxygen is removed in those ways then fish will die.
It is time for everyone to fight to the death to save the rivers from dying.