Tailings
U.S. Bureau of Land Management photo by Steve Moore. (fair use policy)
Gold dredgers systematically dug up all the gravel in this small Idaho valley, washed out its small fraction of gold, and dumped the tailings behind them. This kind of hydraulic mining can be done responsibly: a catchment pond settles out the clay and silt to protect the downstream environment, and the tailings can be graded and replanted. In a large land with few inhabitants, some degradation can be tolerated for the wealth that's created.
But in times past, as during the California gold rush of the 1850s, there was plenty of irresponsible dredging. The rivers of the Sierra Nevada and the Great Valley were so severely disturbed by tailings that navigation was hampered and farms failed after being flooded with sterile mud. The state legislature was ineffective; finally a federal judge banned hydraulic mining in 1884. Read more about it on the Central Pacific Railroad Photographic History Museum site.
A recent study concluded that all the work we do in moving rock, water and sediments around makes humankind a significant geomorphic agent—just like rivers, volcanoes and the rest. In fact, human energy is more effective than all the world's erosion at the moment.
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