Impacts of suction dredging on rivers, fish and aquatic communities
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Fact sheet: Download Rogue Riverkeeper's fact sheet on suction dredging, with a quick overview of the issues and impacts, with citations to peer reviewed scientific literature.
During the Gold Rush of the 19th century, miners could strike it rich by panning the right streams for gold using very simple equipment. Technologies quickly developed to access more gold, including hydraulic mining, hardrock mining, large mechanical dredging and the use of mercury. The suction dredge was developed in the 1960s as a type of portable underwater vacuum to suck up river sediments and then filter out the gold.
In 2012, there were approximately 2,000 miners registered to use suction dredges in Oregon’s streams, almost all of whom were working in southwest Oregon’s rivers, a 200% increase from the previous year. In California, prior to the current moratorium, approximately 3,200 suction dredge permits were issued each year.
The basic configuration of a suction dredge is a floating system, or sluice box, attached to a suction hose that sucks up the river bottom. The stream sediments are run through the sluice box, gold is filtered out and the sediments are discharged back into the stream. The size and power of a dredge can vary, with motors typically ranging from 2 to 50 horsepower and the vacuum nozzle ranging from 2 to 10 inches. As with all in-stream mining, suction dredging impacts streams, fish and aquatic life. Here is a brief description of some impacts.
The River (hydrology and geomorphology): Suction dredging of any scale or magnitude instigates some degree of erosion, transport and deposition of sediment, along with potential modification of the channel form and stream flow hydraulic conditions. These impacts can include the physical mobilization of alluvial sediments (material deposited from running water) and the removal of such sediment from bars or riffles that are used as habitat for aquatic species. Miners also move and re-locate cobbles and small boulders to access finer sediments and thereby alter the channel bed and aquatic habitat.
Water Quality (and toxicology): People who are suction dredging often develop campsites for extended periods of time. Impacts associated with encampments—including development of access roads/trails, clearing of native vegetation and soils, accidental spills, and waste discharges such as fecal matter and synthetic materials—can have deleterious effects on water quality, fish and other aquatic life.
Suction dredges are powered with a dredge-mounted gasoline engine that must be refueled with gas and engine oil. Refueling and maintenance of these machines can result in discharges of fuel and oil to water and soil where it may be transported by rain or directly discharged into streams. Some miners re-fuel their dredge while the machine is floating in the stream, thereby increasing the instances of gas and oil spills into the waterbody.
Turbidity and suspended sediment are associated with negative effects on the spawning, growth, and reproduction of salmon and steelhead. Human activities in the Northwest, including logging, grazing, agriculture, road building, urbanization, commercial construction and mining—including suction dredging—contribute to periodic pulses or chronic levels of suspended sediment in streams. When sediments are sucked up in a dredge and spit back into the stream those sediments are suspended in the water column, increasing turbidity. Suspended sediments may affect salmonids by altering their physiology, behavior, and habitat, all of which may lead to physiological stress and reduced survival rates. The amount of turbidity caused by suction dredging depends on many factors, including substrate characteristics, dredge motor and nozzle size, river conditions and stream flow and the skill of the dredge operator.
A major concern for water quality is waste discharges and the re-suspension of sediments that are contaminated. Dredging targets the known gold-bearing streams where historic mining took place in the 19th and 20th centuries. Historically, liquid mercury was used in gold processing and that mercury was often discharged into streams and deposited into stream sediments. Mercury is toxic and bioaccumulates in the food chain of aquatic organisms, terrestrial wildlife and ultimately humans. Other natural and human-generated contaminants such as trace metals or synthetic organic compounds (e.g. pesticides) can be present in river sediments where suction dredging occurs. Dredgers often target deep sediments (those too deep to be available to scour under winter flows) and therefore mobilize sediments that would not normally be mobilized. Such deep sediments may have elevated levels of mercury (and other trace metals including copper, lead and zinc) and could be released into the water column by dredging, and transported downstream.
Fish and other Aquatic Life: Suction dredging has significant impacts on fish and amphibians, especially during reproduction. Spawning is a stressful period and fish are highly vulnerable to disturbance during this time. High levels of human activity, including swimming and boating can cause reduced fish egg success, so suction dredging most certainly can impact the reproductive success of fish. Lamprey are particularly sensitive to disturbance.
Fish, including salmon, steelhead, sculpin, minnows, suckers, lamprey and trout utilize small gravel substrates for spawning. Dredging can reduce substrate embeddedness (the degree to which gravel, cobble, boulders and snags are covered or sunken into the silt, sand or mud of the stream bottom) and therefore impact spawning habitat. Although dredge tailings may be attractive to spawning fish, studies have shown they are less suitable for successful spawning than natural gravels, as the tailings are unstable and experience increased scouring.
Fish eggs, fry, larvae and juveniles can be sucked up (entrainment) and displaced by suction dredging, which can cause death. If they do survive, they can experience higher mortality rates due to increased predation, injuries, physiological stressors, disorientation, abrasions and infections. Mollusks and amphibians are important parts of stream ecology and can also suffer death from suction dredge entrainment, a change or reduction in food and burial in dredge tailings.
The increased suspension of sediments in the water column as a result of suction dredging can negatively affect life stage developments of fish and amphibian species by reducing the availability of intra-gravel water flow and dissolved oxygen, which is critical to successful egg development.
Suction dredging can alter fish and invertebrate behavior, including clogging respiratory structures, reducing feeding rates, disrupting courtship displays and spawning behavior and reducing hatching rates in fish.
Suction dredging can result in the creation, alteration or destruction of pool habitat. Dredging can alter or destroy pools by redistributing stream substrate in a manner that would destabilize bed form, or by filling in a pool with dredge tailings. Dredging can affect the ability of a stream to provide thermal refuge, which is critical for salmon and steelhead reproduction and success.
Suction dredging can impact stream ecosystem composition, diversity and resiliency. Stream bottoms, or benthic communities, are important components of a stream ecosystem because they form the foundation of the stream’s food web. Changes in benthic community composition and productivity can affect higher trophic levels (fish and amphibians) and stream processes (organic matter processing). Frequent disturbance of the benthic community may keep them in early stages of development, thereby affecting the composition and function of this community and impacting salmon and other fish that depend on a healthy benthic community.
Streambanks support riparian vegetation, which is important to aquatic food web dynamics, regulation of stream hydraulics and temperature and storage of alluvial sediments. Streambank destabilization and erosion is a primary source of pollution in a watershed and excessive erosion caused by human activities significantly degrades aquatic habitat. Impacts of suction dredging on streambank stability can be significant.
In addition to the direct impacts of suction dredging, there are deleterious impacts from associated activities, including moving in-stream boulders and wood to facilitate dredging, temporary or permanent flow diversions and impoundments and the transport of aquatic invasive species from one stream to another via equipment. Additional concerns about suction dredging include impacts to cultural resources, aesthetics, noise, recreation and fire hazard.