Many types of fish and other aquatic animals cannot survive. Organisms such as bacteria and viruses enter waterways through untreated sewage in storm-water drains, run-off from septic tanks, and from boats whose owners dump sewage into the water. When material such as leaves and grass clippings, and waste from farm animals enters the water, it rots and breaks down and uses up the oxygen in the water.
Pollution of waterways is also caused when silt and soil washes off ploughed fields, construction and logging sites, and from river banks when it rains. This is harmful to the fish and other invertebrates that live in water because it make it hard for the animals to breathe. The plants clogs the waterways, use up oxygen in the water, and block light to deeper waters. When fertilisers are washed into rivers and streams the nitrates and phosphates cause excessive growth of water plants. Nutrients in fertilisers cause excessive growth of plants and algae which takes oxygen and light from water animals Fertilisers contain nutrients such as nitrates and phosphates that help plants to grow. Even the insects that live on the surface of the water are badly affected. Animals and plants living in the water can't breathe, the oil coats the feathers of water birds, and the fur of animals that swim in the water, causing them to become sick and, if there is a great amount of oil on their bodies, to die.
Oil forms a thin layer on top of water and act like a lid on the surface and the water.
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