It was a maaaadhouse in California when gold was first discovered!!!
Nah, that's a bit over the top, though it was.
What about:
'The population in parts of California increased at an exponential rate as people from all parts of the world arrived with the misguided idea that gold would be easily found.'
It wasn't really a rush. The same thing happened in Australia in the 1850's. It was more of a rapid population increase as compared to previous years.