Not to be confused with the suction dredge used by many gold prospectors today, a bucket line dredge is a giant gold processing machine that can move literally tons of material in a short amount of time. They were used extensively throughout the world during the early gold rushes in the U.S. and around the world. Today, most of them are out of commission and lay dormant. While they were extremely efficient and cost effective to operate, they caused irreparable damage to riparian areas. Today most are just relics of the past in various stages of decay.
The basic design of a bucket line dredge is relatively simple. The front of the dredge is comprised of a series of huge metal buckets called the boom. Picture a series of metal scoops about the size of a bathtub, designed to go around in a continuous loop similar to a huge chainsaw. The boom is angled down into the earth and digs up a continuous supply of gravels to be processed.
The material is brought into the main part of the dredge called the hull. The hull is basically a floating factory where several men can work inside of it. As the buckets drop material into the hull, it is sifted through a sieve to remove the larger non-gold bearing materials. This material would consist of the large boulders, rocks, sticks, logs, and any other debris that gets picked up by the boom. One fun fact about this system is that the occasional large nugget was separated out by the sieve and never processed. While it may seem that this was a terrible design flaw, the fact is that large nuggets are so rare, that it is not efficient to process the extra tons and tons of material that would need to be worked just to recover the occasional large nugget. So there are still some monster gold nuggets that were discharged by the dredge and waiting to be found!
The smaller material that falls through the sieve gets processed through a sluice system and the gold is recovered using standard recovery methods. Waste material, both the large unprocessed material and the smaller processed gravels are moved out of the back of the dredge by conveyors. As the dredge moves forward, this waste material is piled out the back of the dredge.
Bucket line dredges are not self-propelled, so they need to be manually anchored in position. This is done using winches and locked in place using a large stake called a spud. Once anchored, the whole dredge and boom is designed to move around to reach all the material in front of it. Once the available material is depleted, the dredge is winched forward and reset in place. They continue onward, zigzagging across the valley floor churning up material for processing. Areas that have been dredged can be easily distinguished by huge piles of waste rock that often go on for miles and miles.
As stated earlier, bucket line dredges were quite hard on the environment. The damage that they incur on riparian areas is generally considered unacceptable, and they have been banned in most countries today. They are still used in some 3rd world countries, and they are indeed efficient at recovering gold. Today you will find old dredges scattered around the world, most of them sitting in the same place that they were parked over a hundred years ago.
You may also be interested in reading about hydraulic mining
, another method used by the early day prospectors.