173 is the natural number between 172 and 174 . It is also a prime number.

173 is a prime number, a Sophie Germain prime, the sum of three consecutive primes (53 + 59 + 61).

It takes around 173 years for all the water in Lake Superior to be "changed"

Sources:

1. http://www.coolquiz.com/trivia/canada/water.asp