The railroads brought new hordes of land-crazy people, and the new Americans moved like locusts across the continent until the western sea put a boundary to their movements. Coal and copper and gold drew them on; they savaged the land, gold-dredged the rivers to skeletons of pebbles and debris.
–John Steinbeck, America and Americans
In describing the rivers as “skeletons of pebbles and debris,” what does Steinbeck imply about the consequences of dredging for minerals?