What is the worst known case of water pollution?

There are no criteria for the "worst" case of water pollution so it is impossible to identify the winner. Contenders for the title would be:

• The pollution of Lake Erie with phosphorous from detergents which ended commercial fishing for many years.
• The release of Mercury from pulp and paper mills that made fish dangerous to eat
• The contamination of the Thames River in England in the 18th and 19th centuries
• The Rhine River contamination in Germany
• The Canals of Venice
• Mercury losses in Japan leading to Minamata disease
• Ohio's Cuyahoga River, which had been compared to "an open septic tank," and frequently caught fire