This story actually begins with a train of all things. My son is obsessed with them. For a belated Christmas gift, we visited the Sumpter Valley Railroad in early summer so he could ride an actual train.
Unbeknownst to me, the huge piles of rocks and dirt surrounding the track were remnants from a gold rush. After the initial gold rush, all that was left was deep river bed gold, but that clearly didn't stop gold hunters. They built a dredge so they could dig up what was left! (Source)
A dredge can literally cut its way through dry land. As the front buckets scoop up dirt, the surrounding river water fills in the void, allowing the dredge to float and continue cutting a path.
The particular dredge we visited was restored and opened up for visitors to witness a piece of history. As you can see, it is massive!
It's estimated this dredge pulled up more than $4 million in gold during the 19 years it operated!