Looks surprisingly low tech / ad hoc.

Yeah, I suspect these dams date from the teens of the last century or so. Nowadays if you use native stone it's for esthetic and environmental considerations; back then it was the cost-effective thing to do!

For some reason I still have trouble believing the age of the development of the West, that such extensive infrastructure was built in the early 1900's. It's difficult to imagine it having been built by hauling rocks either with horses and wagons, or them having built a steam railroad to haul it.

Well, no EIS in those days-- ;)

But yeah, at the time of the First World War and the dawn of the car culture there still would have been living people who remembered the Gold Rush. For all the talk about rapid technological change in the present, I've seen it pointed out that the greatest changes occurred between ~1890 and 1920. By 1920 we recognizably have the modern world.