Suction Gold dredging is another mining method commonly used In this drainage. The suction dredging season generally is from mid June until late September. Commercial operators dredge between 60 and 90 days per season. Most large suction dredge operations employ one submerged operator and a person on the dredge. The submerged operator works in deep water upstream of the Gold dredge. Suction is produced by pumping water through a nozzle at high pressure. The dredge sucks gravel, sand, and rocks off the river bottom and discharges the slurry into a sluicebox. A flexible plastic hose with an intake nozzle is directed at the riverbed deposition while the floating dredge train downstream. The dredge works much like a vacuum cleaner as the operator moves the nozzle in a sweeping motion upstream. Large materials are worked around or moved aside by the submerged operator to avoid plugging the hose. Gravel and water are washed through a sluicebox on the dredge, and gold and black sand are concentrated in the riffles.
Suction Gold dredges discharge tailings and process water directly into the live stream without water water treatment. A sediment plume may extend downstream 25-100 ft but is quickly dissipated. Sand and cobbles are mixed and deposited on existing tailings, effectively reducing downstream migration of most settleable and suspended solids. For environmental regulatory purposes, most suction dredges fall into the category of “non commercial mines”, which are those that process less than 20 cubic yards per day. Tailings left by the dredge vary in width and height, and partially fill the cut where materials were removed by the nozzle. In most areas, dredgers work in deeper water where tailings are not visible, and are not exposed during low water. Suction Gold dredge tailings are not reclaimed by the operator during or after operations. High water generally redistributes the tailings, usually within a year of dredging, depending on the frequency and severity of high water events.
Suction dredges are operated in open, moving water near the shore within the first 25% of the available water column’s width, where the machine and the diving operator are surrounded by water. As water levels drop, dredging operations move toward midstream Water levels, clarity, volume, and velocity dictate where and when the suction dredge can operate. The submerged Gold dredge operator must have good visibility to mine efficiently and safely. Some areas can only be mined during periods of low water.
As the submerged operator follows the gold bearing deposit depth of the cut, material size, sweep width, water depth, and velocity all control the total area worked. For estimates of area and volume of riverbed materials processed by suction dredges. The high shore area below the vegetation line is usually dredged during breakup in June or other high water events. Mining above the vegetation line along the shore has occurred in the past, but is currently reduced by dropping water levels and miner education. Some recreational miners visiting the river for a limited period may find water levels too high to mine the lower shore-lands and may resort to mining the upper shore and Into the vegetation.