Question

What would MOST LIKELY happen if Londoners started putting household waste in the Thames again?

Answer 1

The water would become polluted

Answer 2

The same thing that happened in the 19th century, "The Great Stink". The name is real and it describes an event from the past which would repeat if they started throwing waste again. The river would become so polluted that disease and contagion would begin to spread. In the 19th century, it was cholera. Now, nobody knows.