The early Forty-Niners panned for gold in California’s rivers and streams, or used "cradles" and "rockers" or forms of placer mining. Placer mining is the mining of alluvial deposits for minerals using water under pressure to mine, move, and separate the precious material from the deposit.

Modern estimates by the U.S. Geological Survey are that some 12 million ounces of gold were removed in the first five years of the Gold Rush.

From 1853 to 1884, "hydraulicking" of placers removed an enormous amount of material from the gold fields, material that was carried downstream and raised the level of the Central Valley by some seven feet in some areas and settled in long bars up to 20 feet thick in parts of San Francisco Bay.

The process raised an opposition calling themselves the "Anti-Debris Association". In January 1884, a United States District Court banned the flushing of debris into streams, and the hydraulic mining mania in California's gold country came to an end.