MINING "LIVE RIVERS"

MINING LIVE RIVERS

By RKC

It has always been well known that the deep and rapid rivers, which flow through auriferous areas, contain amounts of gold which would well repay the trouble of recovery.

Many and varied methods have been attempted to mine river deposits, and in recent years the new technology of eductor dredging has made "river mining" a relatively simple process. Certainly when compared with the methods used by the old-timers at the time of the great gold rushes in the late 1800s, eductor dredging is a very simple process.

During the 1800s no other branch of mining was subject to more uncertainty than was "river mining". Probably more effort was expended in unsuccessful attempts to mine rivers than in any other mining activity of the time. To mine a river was entirely a problem of restraining and handling large volumes of water ... and the greater the volume, the more difficult it was to work!

The miners of those days distinguished the difficult to mine, deep and fast flowing rivers, by referring to such watercourses as "live rivers". And in their attempts to mine these "live rivers", the miners constructed dams to divert the river flow so the bed could be exposed for mining. There were two damming methods in common use. The construction of wing dams was one, and the other involved diverting the entire flow into a flume (a conduit for carrying water), or tunnel.

Wing dams were temporary structures built to expose part of a stream for mining, by damming out the river. And to construct a wing dam, a dam wall would be built to extend from the riverbank out into the stream as far as practical while still permitting the free flow of the river. The wall would then be turned, to run downstream and parallel to the riverbank. To then isolate the section of riverbed to be mined, the dam wall would be turned back to the riverbank. The resulting rectangular, or triangular or crescent-shaped, section of riverbed would then be dewatered by the use of a California pump. The pump would also be used during mining to get rid of the water that would leak through the dam wall. Wing dams were only practical in rivers shallow sections, and were often destroyed during flooding.

To divert the entire flow it was necessary to construct what was called a "head dam" across the width of the river from bank to bank. This "head dam" raised the water level to where the flow diverted into an artificial channel. The nature of the artificial channel varied according to the topography surrounding the river, and where the river ran in a more or less straight line, the artificial channel was a flume. At bend in the river, the artificial channel was often a cutting dug into the riverbank to bypass the river bend.

Today it is not uncommon to see channels, or tunnels, in areas of past mining activity. These cuttings are often the only remaining evidence to indicate that the bed of a river had been mined.

River mining could only be carried out with safety during the dry months of the year when the water was low, and not subject to sudden flooding. The season was always an uncertain quantity and it was common for only a short period to elapse before rising waters swept away the dams to shut down the workings.

In California during the late 1800s, river mining was carried out on a scale greater than in any other gold bearing district in the world. And the Feather River was the location for the largest of California's 'damming and fluming' operations. The preliminary work alone on one Feather River claim comprised the building of 16 kilometres of road, and the construction of three separate camps to provide accommodation for up to 300 miners. The head dam on this claim had a width of 43 meters, and the flume, which was constructed using cement and rock, had a length of 1,828 meters and a width of 12 meters.

An even larger dam and flume was constructed on another claim a short distance upstream. The conditions on this claim were somewhat different in that the river was confined between steeper banks. In place of a cement channel, a timber flume was built suspended at an elevation of 27 meters above the riverbed.

The head dam was 133 meters wide, and had a height of nearly 16 meters. The materials used in the dam construction comprised 20,000 tons of rock and 13,716 meters of timber. Over 381,000 meters of timber was used to construct the flume, which ran for a length of 1,188 meters. This flume has the distinction of having carried the largest amount of water ever by a mining flume.

As a precaution, each board of timber used in the flume was marked, and everything so arranged, that in the event of an unexpected flood the entire flume could be removed within 48 hours. At the dam wall the river depth was 15 meters, and the water backed up for 11 kilometres. As many as 500 men were employed on these claims during the season of low water.

Even though these constructions were of a scale greater than any previously attempted, the flumes were still not big enough to carry the river during winter. For more than six months each year, water flowed over the dam wall and along its natural bed. Mining was possible for no more than five months each year and even during the dry months could be halted by a sudden downpour.

In terms of capital invested, and length of river exposed for mining, the most important river mining enterprise ever undertaken in California was attempted in the North Fork of the Feather River. At a bend in the river the miners dug a tunnel over 3,657 meters long, which took four years to construct. A dam 30 meters wide and four meters high was used to divert the river through the tunnel. On first use it was found that the tunnel could not accommodate the water flow, and another two years were spent enlarging the tunnel.

Once the river was successfully diverted, mining commenced. However, on exposing the riverbed it was found that the shallow ground parallel to the riverbanks had already been mined in many places by earlier miners using wing dams. It was then found that the bedrock in the deeper un-worked centre of the river was hard and smooth with little gold. Further problems were encountered in the following seasons, and this claim eventually amounted to nothing more than a drain on the finances of the investors.

River mining in California was severely disrupted during the years 1857-58 when it became a common practice to dump the tailings from hydraulic sluicing mines into the rivers. In the miners rush to exploit the more easily worked auriferous ground in the riverbanks and terraces, large sections of rich un-worked river channel were buried beneath thousands of tons of tailings.

An extreme example of this practice occurred in the Bear River. This river was filled with tailings to a depth of nearly 24 meters in the centre, and its banks covered to such an extent that the tall pine trees previously far above the river were gradually covered until only the top branches appeared above the current. These tailings had an average depth of 91 meters, a width of 22 meters, and a length of 16 kilometres. This area was estimated at the time to contain 33 million cubic meters of tailings.

Scattered throughout the 33 million cubic meters, and through the tailings from most other hydraulic mines, were significant amounts of gold. Hydraulic mining during the late 1800s was often carried out on the basis that it paid better to treat large quantities of gravel with a great loss of gold, than a small quantity with a small loss. The coarse gold was most likely caught fairly easily, but a great deal of the fine and flaky gold, along with some mercury/gold amalgam, was carried with the tailings and disseminated throughout the rivers. It was generally believed by the miners of the time that because the gold was randomly scattered it could not be reprocessed profitably, and was lost not only to their own generation of miners, but also to succeeding generations.

What they did not realize was that over the following 100 years the natural process of regular flooding would re-concentrate the gold into 'pay-streaks'. This process was assisted by the lose-packed nature of the tailings.

Today modern gold dredgers are recovering the very same gold thought lost forever by the old-timers … the method of recovery being one that the miners of the 1800s could not have imagined!

California rivers such as the Trinity, Yuba, American and Klamath, were all mined by the old-timers, yet all of these rivers still carry gold which is payable when a modern eductor dredge is used as the means of recovery. The Klamath River in particular is currently one of the most heavily dredged rivers in the world today. During the summer months it is possible to view dozens of eductor dredges working in a single area. These dredges work legally on registered river claims, which cover much of the 128-kilometer long river. The fact that the Klamath is currently producing payable gold for modern gold dredgers is particularly interesting when the early history of river mining in the area is examined.

For the old-timers, the conditions for river mining in the Klamath were as favourable as in any of the larger California Rivers, and since gold was first discovered it was consistently mined. And although this river carried a large volume of water, the water was generally of not great depth and its velocity was manageable. Also, hydraulic mine tailings did not accumulate to cover the riverbed. The season for river mining was more then 8 months, beginning about the middle of March and ending in early December. This season was the longest of any in California.

River mining was so continuous, and carried out in such an orderly fashion, that the locations of the worked and un-worked sections were well known at any time. In many other California rivers, and certainly in most Australian rivers, mining was so haphazard and intermittent that the miners could never be sure if a likely looking area had already been mined.

These days we can mine the rivers more efficiently than ever before to recover gold on a scale, which can equal, or even surpass, the amounts of gold produced during earlier days. The modern equipment that is available varies from giant bucket-wheel suction dredges that can mine the bulk yardage low-grade deposits located in the lower reaches of rivers, through to eductor dredges.

Modern eductor dredges vary greatly in size, from the 3 and 4-inch recreational dredges used by hobby fossickers who work exposed crevices in shallow streams, to the 5-inch prospecting dredge, and the new generation of 6-inch, 7-inch, 8-inch, and 10-inch production dredges used by professional miners. Eductor dredges are presently being used not only in their country of origin, the USA, but also in countries as diverse as Canada, Papua New Guinea, a few of the small Pacific Island countries, New Zealand, some African and South American countries, and even China.

Professional dredging is carried out on a massive scale in South America. And along Brazil's Maderia River during the 1980s, more than 1,000 dredges operated daily, to produce an estimated four tons of gold a year.

New Zealand has a growing eductor dredging industry, which is providing increasing employment opportunities for individuals. And apart from eductor dredging, there are a significant number of small-scale gold mining operations, which utilize a uniquely New Zealand type of excavator-fed trommel processing plants, to mine rivers. And on the West Coast of the South Island, one of the biggest bucket dredges of its type in the world operated for many years during the 1990s in the Grey River. These plants are permitted to operate under a system of river mining claims (mining permit) that specify very strict environmental controls, which ensure there is no detrimental environmental impact arising from their use.

In Australia there is no doubt that payable gold exist in many of the rivers of Queensland, Victoria, New South Wales, and Tasmania. Also the equipment to mine these deposits is readily available. However, river mining is not being carried out, except as an unauthorized mining activity. The Mining Acts of these States make it difficult to gain approval for any mining to be conducted in the wet bed of rivers. For example, in Queensland the operation of an eductor dredge contravenes certain sludge abatement and rehabilitation provisions of the Mining Act.

What is required in Australia, is the introduction of a riverbed mining claim system designed specifically to cater for eductor dredging. And with proper controls, such as restricting all mineral recovery to below water level, mining can be carried out in a manner, which is environmentally sound. And with the introduction of a suitable river mining claim system, eductor dredging could rank on a scale with the opal mining and sapphire production industries as a viable small-scale rural industry.

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