TURBIDITY CONCENTRATIONS MEASURED BELOW OPERATING
SMALL-SCALE GOLD SUCTION DREDGES
Joseph C. Greene, 05/26/2012
The scientific data regarding the effects of suspended sediments on fish have been derived from laboratory
experiments, observations of natural systems, and in situ experiments. Due to the variable approaches and
methods used in these studies, the generalization and extrapolation of their results to specific areas of concern
must be made with caution.
Total suspended solids (TSS) exposure frequency, duration, particle size, particle type, and fish life stage are
critical in determining the magnitude of physical effects on fish. Research has found that duration of exposure
plays a more dominant role than TSS concentration (Anderson et al. 1996).
While suspended sediments are often the main contributors to turbidity, other non-sediment sources that affect
light transmission (that is, natural tannins and algae) can also influence turbidity (CH2M Hill, 2000).
Field measurements of changes in turbidity and suspended sediment below suction dredges indicate minor,
localized effects. For example, turbidity was 0.5 NTU upstream, 20.5 NTU 4 m downstream, and 3.4 NTU 49 m
downstream of an active dredge on Canyon Creek (Hassler et al. 1986).
On Butte Creek and the North Fork of the American River where ambient turbidities were <1 NTU, maximum
turbidity 5 m downstream of active dredges reached 50 NTU but averaged only 5 NTU (Harvey 1986). One must
remember sediment plumes below suction dredges are intermittent not continuous.
In Gold Creek, Montana, suspended sediment was 340 mg/L at the dredge outflow and 1.8 mg/L 31 m
downstream of an active dredge (Thomas 1985).
Turbidity and total filterable solids (TFS) both displayed an increase below the dredge during operation of the
dredge, turbidity increased from values around 1 NTU upstream of the dredge to values of approximately 25
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NTU immediately downstream of the dredge. The elevated turbidity declined rapidly downstream and by 160
m (= 525 ft) turbidity had returned to values measured upstream of the dredge.
An excellent illustration is the measured turbidity plume behind an 8-inch dredge operating in the 40-Mile River
Alaska (Figure 1). Each of the 5 turbidity charts are measurements taken at different distances below the
dredge and each is a turbidity assessment across the entire width of the river. The charts illustrate that the
turbidity plume expands little in width from what it was just behind the dredge. Furthermore, the turbidity
downstream of this 8-inch dredge had dropped down to 20 NTU just 66 feet downstream.
On the Fortymile River, 6- and 8-inch dredges showed that turbidity was reduced to background levels within
250 feet. It is expected that small dredges would have even less impact on the downstream receiving water
quality" (Prussian et al., 1999), such as those used in the Klamath River system.
Wanty et al. (1997) reported turbidity values of 19 NTU 30.5 m (100 ft) downstream of a 10 inch dredge located
below Wilson Creek on the North Fork Fortymile River. Values returned to near background levels (3.7 NTU)
within the next 30.5 m but remained slightly above background levels (2.2 - 2.3 NTU) as far as 150 m
downstream (furthest sampling transect).
The Oregon Department of Fish and Wildlife (1980) reported that very little turbidity results from normal use of
smaller suction dredges (4-inches or less) in stream gravels. The majority of heavy suspended solids settle out
within a few yards.
I was surprised to find a statement by the U. S. Army Corps of Engineers (1994) which came out against the
positions held then, by environmentalists and the regulatory community. The purpose was to show the findings
of de minimus (inconcequential) effects on aquatic resources for 4-inch and less suction dredges and hand
mining. The Corps summarized the situation in 1994 as follows: “Four-inch and smaller dredges have
inconsequential effects on aquatic resources. This is an official recognition of what suction dredgers have long
claimed; that below a certain size, the effects of suction dredging are so small and so short-term as to not
warrant the regulation imposed in many cases.” The reports consistently find no actual impact of
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consequence on the environment, and so almost always fall back to the position that potential for impact
exists.
“The U. S. Environmental Protection Agency, has ignored this concept, although numerous studies, including the
EPA’s own 1999 study Royer, Prussian and Minshall (1999) of suction dredging, repeatedly and consistently
support the Corps findings of de minimis (inconcequential) effects even though the dredges used in the study
were 8- and 10-inch.”
“The regulatory agencies should be consistently and continually challenged by the dredging community to
produce sound, scientific evidence that support their proposed regulations. To regulate against a potential for
harm, where none has been shown to exist, is unjustifiable and must be challenged (U. S. Army Corps of
Engineers, 1994).”
The scientific data regarding the effects of suspended sediments on fish have been derived mostly from
laboratory experiments and in situ experiments with few observations of natural systems, Due to the variable
approaches and methods used in these studies, the generalization and extrapolation of their results to specific
areas of concern must be made with caution. Generally, the “real world” turbidity effects on freshwater fishes
will not match up with these controlled experiments because the test fish are trapped within the experimental
plume for the full duration on the test. In the real world the fish downstream from an operating small-scale gold
suction dredge can move into, and out of, the plume while feeding.
The TSS influences on fish reported in the literature range from beneficial to detrimental. Turbid water can be
beneficial in somewhat low concentrations and act as cover to protect fish from predation. Fish that remain in
turbid water experience a reduction in predation from piscivorous fish and birds (Gregory and Levings 1988).
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Results of turbidity surveys behind an operating 10-inch suction dredge. All numbers shown are in NTU, or
nephelometric turbidity units, the standard unit of turbidity. The right bank of the river is off the edge of the
figure. The approximate shape of the plume is shown in gray. Note that the figure is exaggerated 5X
horizontally, so the plume is actually much narrower than it appears in the figure. (Gough et al., 1997)
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Figure 1. Turbidity plume in stream cross-section
Suction dredge equipment, used by modern-day gold miners on the Klamath and other
California rivers, dirty the rivers and disturb the spawning grounds of already imperiled
fish. Photo: courtesy Craig Tucker, Karuk Tribe
The above photograph does not support the claim stated by Craig Tucker and the Karuk Tribe. It is clear that
the turbidity plume dissipates in very short distance and the turbidity is restricted to a very narrow plume relative
to the actual width of the river. The spawning gravels in the Klamath River system are severely embedded due
to sediment run-off into the streams from deforested lands primarily from the many major forest fires that have
occurred in the region in recent years. Remember the logging industry has been destroyed based upon saving
the spotted owl. So, no major logging has occurred for years. The suction dredge breaks up the cemented
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surface gravels and makes them available for current and future spawning sites. The hole dredged into the
overburden serves as refugia for the fishes during the hot dry summer.
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Another illustration of the narrow plumes downstream of the small-scale gold suction dredges. Turbidity
plumes, usually, do not cover wide areas of the stream and they are not continuous or consistent in sediment content
Based just on the data shown above the fish would have been required to be trapped in the sediment plume
for 24-hours a day for 7-weeks to 4-months for a small-scale gold suction dredge plume to cause any harm.
This scenario does not realistically describe the operations of such a dredging system. Most dredgers seldom
work more than 5-hours a day to allow time for cleanup of the day’s collected materials.
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Based just on the data shown above the fish would have been required to be trapped in the sediment plume
for 24-hours a day for 1 to 6-days for a small-scale gold suction dredge plume to cause any harm. This scenario
does not realistically describe the operations of such a dredging system. Most dredgers seldom work more
than 5-hours a day to allow time for cleanup of the day’s collected materials.
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Based just on the data shown above the fish would have been required to be trapped in the sediment plume
for 24-hours a day for 7 to 8-days, at turbidity concentrations not reached from dredging, for a small-scale gold
suction dredge plume to cause any harm. This scenario does not realistically describe the operations of such a
dredging system. Most dredgers seldom work more than 5-hours a day to allow time for cleanup of the day’s
collected materials.
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REFERENCES
Anderson, P.G., B.R. Taylor, and G.C. Balch. 1996. Quantifying the Effects of Sediment Release on Fish and their
Habitats. Canadian Manuscript Report of Fisheries and Aquatic Sciences No. 2346, Department of
Fisheries and Oceans, Vancouver, B.C. and Winnipeg, Manitoba, 1996.
CH2M Hill, 2000, Suspended Sediment Effects on Fish, A Literature Review- Appendix A.
Gough, L., Day, W., Crock, J., Gamble, B., and Henning, M., 1997, Placer-Gold Mining in Alaska–– Cooperative
studies on the effect of suction dredge operations on the Fortymile River: U.S. Geological Survey Fact
Sheet FS-155-97. The USGS website: http://www.usgs.gov/
Gregory, R.S., and C.D. Levings. 1998. Turbidity Reduces Predation on Migrating Juvenile Pacific Salmon.
Transaction s of the American Fisheries Society 127: 275-285.
Hassler et al, 1986, Impacts of Suction Dredge Mining on Anadromous Fish, Invertebrates and Habitat in Canyon
Creek CA.
Harvey, 1986, Effects of Suction Gold Dredging on Fish and Invertebrates in Two California Streams.
Newcombe AND Jensen, 1996, Channel Suspended Sediment and Fisheries, A Synthesis for Quantitative
Assessment of Risk and Impact
Oregon Department of Fish and Wildlife. 1980. Recreational mining can be compatible with other resources.
Salem, OR.
Prussian, A. M., Royer, T. V., and G. W. Minshall. 1999. Impact of suction dredging on water quality, benthic
habitat, and biota in the Fortymile River and Resurrection Creek, Alaska. Final Report. For the U. S.
Environmental Protection Agency, Region 10, Seattle, WA. 72pp.
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Thomas 1985. Experimentally determined impacts of a small, suction gold dredge on a Montana stream. North
American Journal of Fisheries Management. 5:480-488.
U. S. Army Corps of Engineers. 1994. Special Public Notice 94-10, September 13, 1994.
Wanty, R.B., B. Wang, and J. Vohden. 1997. Studies of suction dredge gold-placer mining operations along the
Fortymile River, eastern Alaska. U.S. Geological Survey Fact Sheet FS-154-97.
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