These substances when it flows into the river. To effect the big is firstly improves the supplements to aquatic organisms The aquatic organisms increase. When the plants and aquatic animals died. Organic matter in the water will cause increased. Organic matter from waste water and that increased. If there are many when being decomposed by anaerobic bacteria contained in the water. It will bring the dissolved oxygen is used at a higher rate than the rate at which the oxygen in the air to melt down in the water. Cause of lack of oxygen. The environment that fits an aerobic bacteria to decompose organic matter. Make water into black smell.