The gold fields of Alder Gulch and Last Chance Gulch were two of the biggest placer gold deposits in the world. Placer deposits consist of loose particles of gold mixed into topsoil, sand, and gravel. Placer gold is near the surface. The easiest way to get placer gold is to wash loads of sand/gravel/etc., the heavy gold particles sink while the lighter dirt and rock and sand particles wash out with the water. This is the method used when panning for gold. As well as when you use a sluice or rocker. In Montana, miners started out by digging the topsoil and running it through sluices. Eventually, however, all of the easy gold had been found. Prospectors either left, or turned to stronger methods of excavation, like hydraulicking.
In hydraulicking, miners would blast hillsides with water, causing high-speed erosion, like a pressure washer. They would direct the slurry of water and dirt through huge sluices. The gold would sink to the bottom and the water and dirt (tailings) would usually get dumped into the nearest creek. Hydraulicking was much more efficient than digging and hauling, but it was really bad for the environment. It was, after all, high-speed mass erosion. Hydraulicking tore apart entire hills, stripping them of topsoil and vegetation. Hydraulicked land was not good for anything else. The tailings-the sand and gravel left over-were either just dumped, leaving giant piles of waste to mar the countryside, or were dumped into streams. That much sediment in the water caused the streams to change course, shift direction, or flood unpredictably. Hydraulicking in the mountains could bury valleys in unusable tailings. The farmers of the Sacramento Valley actually sued miners on the basis that hydraulicking was ruining their farms. In Montana, you can still see evidence of extensive hydraulicking around both Bannack and Virginia City, as well as many of the other mining ghost towns in Southwest Montana.