Above. Placer operation on the Yuba River using a suction dredge.
Did you ever wonder what the real studies say about suction dredging, but didn't want to have to read through them all. A summary of every study we could find is provided here and links to the actual study. You can see our full list of suction dredging studies on the main research page.
Although technically not a study, this is a report of a senior fisheries biologist from the California Department of Fish and Wildlife (CDFW). Responding to complaints from the Karuk tribe CDFW, in conjunction with the USFS sent two senior fisheries biologists to the Salmon River on the last day of dredging season in 2003 to evaluate the effects of an entire summer of dredging and whether these effects could be impacting salmon.
At the conclusion of the on-site evaluation Mr. Dennis Maria, the fisheries biologist concluded:
"The dredge holes created the only discernable juvenile rearing habitat that I could see. This rearing habitat consisted of clean unimbedded cobbles that covered the dredger pool substrate."
"There exist documented instances that unstable spawning gravel mounds created by dredgers below dredge holes have been used by anadromous salmonids only to be lost by high winter flows washing these gravel mounds away. My files indicate little, if any, spawning occurs in this reach of the Salmon River. Peter Brucker, who has been involved with numerous spawning surveys on the Salmon River over the past number of years (>10 yrs.) agreed this reach of the Salmon River is not typically used for spawning.
"...it is unlikely that the current dredging impacts will significantly or substantially harm anadromous salmonid spawning habitat or juvenile salmonids within this reach."
"In fact, for an area which had been dredged all summer long, I saw relatively innocuous disturbance of the existing habitat."
"I saw nothing that would be considered a violation or that would have a significant impact to the fishery or significantly negatively impact the overall biotic community of the Salmon River. I would estimate that the amount of dredger disturbance on the mainstem Salmon River by New 49er members represents at most about 2 to 3% of the entire mainstem Salmon. Nearly most of the disturbed areas we saw during our tour were in areas not suitable for spawning."
The 2012 California SEIR reviewed all available suction dredging studies and found all studies agreed turbidity from a suction dredge was localized and temporary and therefore an insignificant effect of dredging. However, they claimed the combined efforts of every dredger across the State would result in a significant increase in turbidity without citing any study which supported this position. The actual studies found:
In this study Harvey evaluated the effects of suction dredging on Butte Creek and the North Fork of the American River. The significant part of Harvey's study was he evaluated the effect of 5" and 6" dredges over the course of two years and evaluated the effects of a single dredge and the cumulative effect of six dredges. His study primarily focused on the effects of small insects in the river.
Harvey found no significant differences in insect populations in areas which had been dredged, and those which hadn't been dredged and found insects rapidly re-colonized dredged areas.
Harvey concluded fish weren't significantly effected by dredging although short term they would move away from the dredge hole, but he found after dredging all eight study fish had moved from their previous location into the dredge hole indicating a 100% preference of the fish for the deeper water dredge hole.
During the experiment Harvey measured the turbidity of the dredge and found in the relatively clear water the turbidity change was noticeable, but dissipated rapdily and didn't appear to effect either the fish or the insects.
"Fish and invertebrates apparently were not highly sensitive to dredging in general, probably because the streams studied naturally have substantial seasonal and annual fluctuations...Along with the rapid temporal recovery of insects seen in this study, these results suggest that suction dredging effects can be short-lived on streams where high annual flows occur."
This report studied the cumulative effects of suction dredging in the Siskiyou National Forest under contract to the U.S. Forest Service.
The Baley study focused primarily on the effect of suction dredging on young salmon and found no significant effect. He concludes:
"Localized, short-term effects of suction dredge mining have been documented in a qualitative sense. However, on the scales occupied by fish populations such local disturbances would need a strong cumulative intensity of many operations to have a measurable effect. Local information reveals most suction dredge miners more or less adhere to guidelines that have recently been formalized by the U.S. Forest Service...Given that this analysis could not detect an effect averaged over good and bad miners and that more powerful study would be expensive, it would seem that public money would be better spent on encouraging compliance with current guidelines than further study."
In this 1982 study commissioned by the California Department of Fish and Wildlife the research evaluated the effects of suction dredging on the environment across the state and with dredge sizes from 2" to 10". Over 270 dredges were studied.
The authors found "...relatively few suction dredge miners are causing negative impacts."
The authors found the presence of mining claims tended to distribute the dredging operations out and resulted in less environmental impact than in areas where there were no mining claims. The most intensive dredging they found was in areas which were open to dredging, but closed to the location of mining claims. "Claimed streams like the North Fork of the Yuba River had a more even distribution of dredges. However, instream dredging effects tend to be localized not cumulative (Harvey et al. 1982)
The authors concluded "Even with the large increase in the number of suction dredge mining operations in recent years, the aquatic and riparian habitat impacts observed on selected streams of the Mother Lode during this study were minimal.
This 1995 study was conducted under contract to the U.S. Forest service in response to an environmental groups lawsuit which claimed suction dredging was harming the environment.
The purpose of the study was to respond to allegations suction dredges harmed various fish including salmon. The study found salmon would spawn in suction dredge tailings if no other suitable habitat was present which could potentially be detrimental to redd survival if there was a heavy winter flow which could re-distribue the tailings. However, they found the selection of suction dredge tailings was only about 10% and they didn't observe any mortality of redds in dredge tailings.
The study found a near 100% mortality rate if salmon eggs were sucked through the dredge hose and a very high mortality rate if the fingerlings were sucked through the dredge hose. The study found virtually no risk to juvenile and adult fish.
Salmon preference for dredge habitat was found to be uncertain. "Depending on existing water depth and velocity, dredging may increase or decrease the availability of preffered habitat for salmonids by altering the morphology of individual channel geographic units."
The study went on to find the deposition of fine silt by a dredge likely has no impact on salmonids and no clear effect on salmon could be determined by the presence of suction dredges in the river, although the report cautioned the location of suction dredges which would block salmon from finding cooler waters could be a concern.
The report found no effects on stream insects consistent with earlier research.
"The three studies which have quantified colonization by benthic invertebrates afer dredging was stopped (Griffith and Andrews 1981; Thomas 1985; Harvey 1986) measured rapid recovery (within 4-6 weeks) in terms of both numbers and species composition."
"Many recreational dredgers operate for less than five hours per day, suggesting that data from studies exposing biota to chronic suspended sediments would not apply to the impacts of dredging."
"Available data indicate that individual dredges need not have significant downstream effects on aquatic biota. Downstream impacts may occur where closely-spaced dredges create the potential for cumulative effects of multiple dredges. The only attempt to measure cumulative effects of dredging on fish and invertebrates (Harvey 1986) suggested that a moderate density of dredges does not generate detectable cumulative effects.
This study evaluated the effects of a 3" suction dredge on fish and insects on four Idaho streams during the summer of 1980.
This study is interesting because it intentionally ran salmonid eggs through a suction dredge to determine mortality. Although no research, anywhere, has shown a suction dredge to actually do this the author of the study wanted to determine what would happen to various fish eggs if they were sucked up by a dredge. The study concluded for some types of eggs there would be a near 100% mortality rate. Although this may be true, there is likely a 100% mortality rate if you lay the same eggs out on the highway. However, the study also found only the un-eyed stage of the eggs is susceptible to mortality from a suction dredge. Once the eggs were "eyed" they had a much higher rate of survival.
This is another study which measured the effects on insects found within the stream and again concluded recolonization by insects is very rapid and there are no significant effects on insect populations. The study evaluated 2,100 mayfly larvae and found only two individuals were injured by the suction dredge. "Aquatic insects were surprising resilent to the effects of entrainment."
The study found the actual amount of material moved differed greatly from advertised rates. When measuring actual material moved by a 3" dredge the author found only .05 cubic meters per hour, but when measured using only pre-classified sands and gravel the "advertised rate" the amount of material moved was .13 cubic meters an hour, or double.
"Turbidity below the dredge in Napias Creek and Yankee Fork was nearly undetectable. A turbidity plume was noticeable only a few meters below the dredge in Napias Creek and no plume was seen in Yankee Fork.
In this study Thomas evaluated the effects of a 3" dredge in Gold Creek near Missoula, Montana.
"The concentration of suspended sediment was greatest at the dredge outflow and decreased rapidly as the heavier particles settled out. Suspended sediment was 1.8 mg/liter 30.5 meters below the dredge, indicating a return to ambient levels...These data indicated that the bulk of the sediment stirred up by the dredging was re-deposited within 6-11 meters of the dredge."
"Deposited sediment decreased exponentially downstream with the distance from the dredging."
"The immediate effect of suction dredging was to reduce the number of all species of insects in the area dredged. The effect was very localized. No significant change in abundance was found downstream from the dredged section for any taxonomic group...The number of insects in the dredged section increased 1 month after dredging, even when the numbers in the control and downstream sections decreased - indicating most aquatic insects find dredged areas to be suitable habitat."
"Intergravel permeability apparently did increase slighlty in the dredged section after dredging. However this difference was not significant."
"The purpose of this study was to assess the impact of one small dredge operated for a relatively short period of time. The effects seemed to be small, very localized habitat modifications that had a minimal effect on the stream community."
The Prussian Study, conducted under contract to the US EPA is the only known study to evaluate the effects on water quality from suction dredging in regards to trace metals. The study also evaluated turbidity and the effects on aquatic insects. This was also one of the few studies to evaluate the effects of what are considered large dredges: an 8" and a 10" dredge.
The concluions of the study include:
"Dredge operations had no discernable effect on alkalinity, hardness, or specific conductance of water in the Fortymile. Of the factors we measured, the primary effects of suction dredging on water chemistry of the Fortymile River were increased turbidity, total filterable solids, and copper and zinc concentrations downstream of the dredge. These variables returned to upstream levels within 80-160 meters downstream."
The results from this sampling revealed a relatively intense, but localized, decline in water clarity during the time the dredge was operating.
The impact of the dredge piles relative to the width of the Fortymile River was small. After one year, dredge piles at Site 1 had largely disappeared following the scouring flows that accompany snow melt.
Macroinvertebrate abundance and diversity were greatly reduced in the first 10m below the dredge at Site 1 relative to the upstream reference site. The abundance and diversity of macroinvertebrates returned to values seen at the reference site by 80-160 meters downstream. Recovery of macroinvertebrate diverstiy appeared to be substantial.
The results from Resurrection Creek indicated that there was no difference in the macroinvertebrate community between the mining area and the locations downstream."
"Effects of suction dredging commonly appear to be minor and local."
This study fundamentally re-looks previous studies and comes to the same conclusions that suction dredges can kill un-eyed salmonid eggs; re-colonization of benthic invertebrates is rapid; and most effects from suction dredging are temporary and local.
This report adds nothing new to the research it only reviews previous research and makes a few recommendations for future studies.
This study was prepared under contract to the U.S. Fish and Wildlife Service. It is one of the most extensive studies at 147 total pages. The authors studied the effects of various sized dredges on Canyon Creek in Trinity County California.
The studies demonstrated that the impacts of suction dredge mining on fish and habitat were moderate at the current level. The impacts were seasonal and site specific. The current regulations controlling dredge aperature and size and season appear adequate to protect habitat, but careful monitoring of mining activity is advised."
This study was prepared under contract to the Washington State Department of Ecology, Environmental Assessment Program. This study evaluated the trace metal impact of suction dredging on the Similkameen River in Washington. It is more thorough than the Fortymile Study.
The study specifically evaluated the effects of suction dredging on levels of arsenic, copper, lead and zinc in the water column. The report concludes:
Results showed that the metals concentrations discharged from small-scale gold dredges are not a significant toxicity concern for aquatic life in the Similkameen River. Although this activity will exacerbate exceedances of arsenic human health criteria, it would take very large numbers of dredges to effect a 10% change in the river's arsenic levels, even at low-flow conditions.
Based on analyzing 14 effluents and 27 plume samples, it appears that small-scale gold dredges have little or no potential to cause exceedances of aquatic life criteria in the Similkameen River.
The metals concentrations measured in gold dredge effluents during the present study were at or below aquatic life criteria. Therefore, criteria exceedances would not be anticipated in the Similkameen River, regardless of the number of dredges operating.
As the name of this study implies the purpose was to determine what happens to mussels if effected by suction dredging.
The study was conducted in two different areas: Mill Creek and the Similkameen River.
On Mill Creek the study found mussels had a 100% survivability rate of being entrained (sucked up) and all mussels were able to extract themselves from burial of up to 40cm of dredge tailings.
On the Similkameen River the study found a 100% survival rate (and recovery rate) of all mussels which were entrained by the dredge. The study involved simulated entrainment and placement on top of a simulated tailings pile. The mussels were observed for one week and all mussels were able to re-orient themselves and begin feeding after six days.
The Similkameen Study also found from 6% to 13% of mussels which were buried under dredge piles could suffer mortality.
At 89 pages this Master's Thesis is another very thorough evaluation of the effects of suction dredging. The study evaluated a twenty mile length of Canyon Creek with dredge sizes ranging from 2" to 6".
"Turbidity and TSS levels decreased with distance below the dredge. Values 50 meters below the dredge were 2 to 3 times higher than that of the control, but at 100 meters below values returned to normal levels."
"Sediment deposited decreased with distance below the dredge, average deposited sediment ranged widely between dredges."
Substrate embeddedness generally increased below dredge sites in the local area.
Minor scour and fill occurred at all dredge sites ranging 6-10%.
There was no statistical difference in young steelhead population counts above the dredge, below the dredge or at control sites indicating the suction dredging operation had no measurable effect.
The normal annual flows at Canyon Creek were adequate to siperse dredge tailing piles and fill in dredge holes. Less than 9% of the holes and tailings from the 1984 mining year were visible at the start of the 1985 season.
"Most streams with mobile beds and good annual flushing flows shold be able to remove the instream pocket and pile creations of suction dredges, although regulated (dams) streams with controlled flows may not.
"In Canyon Creek, several spawning surveys located approximately 60 salmonid redds in the study area, but none were within a dredge tailing pile.
"Suction dredge mining did not appear to influence the locations of adult anadromous salmonid summer-holding areas."
"Although distinct to even the most casual observer, dredge plumes in Canyon Creek were probably of little direct consequence to fish and invertebrates...In general, dredge turbidity plumes were highly localized and occurrred during midday which is not a peak feeding peeriod for steelhead."
Fish living space may be reduced within the first few meters below the dredge, but just upstream a new pool is created by the cone-shaped dredge hole. During the study, young steelhead, dace and suckers were observed in active and abandoned dredge holes.
"A high level of suction dredging was evident in Canyon Creek, but adverse effects on aanadromous fish habitat were minimal to moderate.
This paper is one of the lesser known, but more thorough evaluations of suction dredging. At 164 pages it is second only to the 2012 California EIR in page length. It was done under contract to the Washington Department of Ecology. It is essentially an environmental impact report of the effects and provides a great single point of reference for suction dredging impacts. This study provides a good review and summary of previous research but adds nothing new to the existing research.