Yuba Gold Dredge no. 17, hard at work, with the Sutter Buttes behind it. This machine floats in a pond of groundwater, digging up gold-bearing gravel and depositing it on land. Over the years, it slowly migrates across the land extracting about 2000 ounces of gold per month for Cal Sierra Development Inc. This is the last operating dredge of its kind in the lower 48 states and the principal producer of California gold today. It was built in 1917 and rebuilt twice, in the 1930s and in 2006.