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Sewage, industrial [http://waterpollution.eu/ water pollution]and chemicals used in agriculture such as fertilizers and pesticides are the main cause of pollution. In the U.S., 37% of lakes and estuaries and 36% of rivers are too polluted for fishing or swimming in most of the year. In developing countries, more than 95% of sewage is thrown into rivers and bays, creating a major risk to human health.

Chemical fertilizers such as phosphates and nitrates used in agriculture are paid in lakes and rivers. These combine with phosphates and nitrates from sewage and speed up the development of algae. [http://waterpollution.eu water pollution] can become "suffocating" because algae are decomposing and depleting oxygen in it. http://waterpollution.eu This process, called eutrophication, can cause death of fish and other aquatic life forms. In the late '90s in the waters of Delaware Bay and Gulf of Mexico thousands of fish died due to development of toxic algae called Pfister forms piscicida. It is believed that the reason for developing this toxic species of algae was saturated with water to agricultural land, excess water reaching the rivers and then into the sea, phosphates and nitrates help algae. Floods cause toxic pesticides and urban and industrial wastes into lakes and rivers.