When floodwater reaches the sea, it can leave a 50-meter-thick layer of brown water, and cause real problems

Over this wet summer, Melburnians and Sydneysiders have had to think twice about cooling off at their local beach. Heavy rainfall has swollen rivers and pumped pollutants, nutrients and murky fresh water far out to sea. Swimmers at Port Phillip Bay beaches are emerging coated in brown goo, while Sydney's seas were contaminated last week.