Why did the california gold rush come to an end

All easy-to-find gold was quickly taken

Explanation:

Miners extracted more than 750,000 pounds of gold during the California Gold Rush. The original Gold Rush as we know it was the 1848 gold strike at Sutter’s Mill in California. And Gold Fever was on. The problem was, this was all easy, surface ore, known as placer gold, stuff that could be found easily in streams. To get it, all you need to do is wash it out of the sand and gravel. Which of course attracted your quick rich schemers. Your typical placer site however would only last a few years. As the placer gold panned out, it was replaced by serious mining.

i'd assume india mainly because of how isolated japan was.   plus it is a whole china away.