Editing Gold mining (section)

=====Dredging=====
{{main|Gold dredge}}
Although this method has largely been replaced by modern methods, some dredging is done by small-scale miners using suction dredges. These are small machines that float on the water and are usually operated by one or two people. A suction dredge consists of a sluice box supported by pontoons, attached to a suction hose which is controlled by a miner working beneath the water. This method is particularly popular in areas where gold is found at river bottoms or submerge deposits'''.'''<ref name="Spence 1980 401–414">{{Cite journal |last=Spence |first=Clark C. |date=1980 |title=The Golden Age of Dredging: The Development of an Industry and Its Environmental Impact |url=https://academic.oup.com/whq/article-lookup/doi/10.2307/968288 |journal=The Western Historical Quarterly |volume=11 |issue=4 |pages=401–414 |doi=10.2307/968288|jstor=968288 |url-access=subscription }}</ref><ref>{{Cite journal |last=Vieira |first=Rickford |date=2006 |title=Mercury-free gold mining technologies: possibilities for adoption in the Guianas |url=https://doi.org/10.1016/j.jclepro.2004.09.007 |journal=Journal of Cleaner Production |volume=14 |issue=3–4 |pages=448–454 |doi=10.1016/j.jclepro.2004.09.007 |bibcode=2006JCPro..14..448V |issn=0959-6526|url-access=subscription }}</ref>
[[File:Yankee Fork gold dredge.jpg|thumb|The Yankee Fork gold dredge in Idaho. It was powered by two 350 horse Ingersoll-Rand, 7 cylinder diesel engines, burning 400 to 500 gallons of fuel a day.]]
Suction dredging can have environmental impacts, moreso on aquatic habitats and water quality.<ref name=":9">{{Cite journal |last1=Harvey |first1=Bret C. |last2=Lisle |first2=Thomas E. |date=1998 |title=Effects of Suction Dredging on Streams: A Review and an Evaluation Strategy |url=http://doi.wiley.com/10.1577/1548-8446(1998)0232.0.CO;2 |journal=Fisheries |language=en |volume=23 |issue=8 |pages=8–17 |doi=10.1577/1548-8446(1998)023<0008:EOSDOS>2.0.CO;2 |bibcode=1998Fish...23g...8H |issn=0363-2415|url-access=subscription }}</ref> Regulations and best practices are often in place to minimize these impacts.<ref name=":9" /> State dredging permits in many of the United States gold dredging areas specify a seasonal time period and area closures to avoid conflicts between dredgers and the spawning time of fish populations. Some US states, such as Montana, require an extensive permitting procedure, including permits.

Some large suction dredges [{{convert|100|hp|kW}} & {{convert|10|in|mm|-1|abbr=on|order=flip}}] are used in commercial production throughout the world. Small suction dredges are much more efficient at extracting smaller gold than the old ''bucket line''. This has improved the chances of finding gold. Smaller dredges with {{convert|2|to|4|in|mm|-1|adj=on|order=flip}} suction tubes are used to sample areas behind boulders and along potential pay streaks, until "colour" (gold) appears.

Other larger scale dredging operations take place on exposed river gravel bars at seasonal low water. These operations typically use a land based excavator to feed a gravel screening plant and sluice box floating in a temporary pond. The pond is excavated in the gravel bar and filled from the natural water table.<ref name=":72"/> "Pay" gravel is excavated from the front face of the pond and processed through the floating plant, with the gold trapped in the onboard sluice box and tailings stacked behind the plant, steadily filling in the back of the pond as the operation moves forward.<ref name=":72"/> This type of gold mining is characterized by its low cost, as each rock is moved only once. It also has low environmental impact, as no stripping of vegetation or overburden is necessary, and all process water is fully recycled. Such operations are typical on New Zealand's South Island and in the Klondike region of Canada.<ref name="Spence 1980 401–414"/>