Short answers, pulled from the story.
Hydraulic mining is a form of mining that uses high-pressure jets of water to dislodge rock or move sediment. In placer mining of gold or tin, the resulting water-sediment slurry is directed through sluice boxes to remove the metal. It is also used to mine kaolin and coal.
The modern form of hydraulic mining was first used by Edward Matteson near Nevada City, California, in 1853 during the California Gold Rush. He directed high-pressure water through hoses and nozzles at gold-bearing upland gravels, using canvas hose that was later replaced with crinoline hose by the 1860s.
A monitor is the giant iron nozzle that produces the pressurized water jet in hydraulic mining. Water was redirected into an ever-narrowing channel, through a large canvas hose, and out through the monitor to wash entire hillsides through enormous sluices. The underground version used in coal mining is called a hydro monitor.
Hydraulic mining was restricted because its sediment buried Sacramento Valley farmland and caused major flooding. In the landmark case Woodruff v. North Bloomfield Mining and Gravel Company, Judge Lorenzo Sawyer ruled for the farmers on the 7th of January 1884, declaring hydraulic mining a public and private nuisance and enjoining its operation in areas tributary to navigable streams and rivers.
Hydraulic mining caused major flooding, raised riverbeds, and released 1.5 billion yards of toxic waste called slickens into the Sacramento River. The slickens carried mercury into San Francisco Bay, which remains dangerously contaminated, with estimates suggesting it will take another century to clear naturally.
Hydraulic mining spread to Oregon, Colorado, Montana, Arizona, Idaho, South Dakota, Alaska, British Columbia, and Georgia, and overseas to Australia, New Zealand, and the Malay Peninsula. It is used today to mine kaolinite clay in Cornwall and Devon, and the East Rand Gold and Uranium Company has run a tailings facility on the South African Rand since 1977.