Suction Dredge Report 2nd Half, Biased yet Informative Options

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Oregon Water Resources Department (WRD)


• WRD grants water rights throughout the state, and is responsible for assuring that
the free flowing character of Scenic Waterways is maintained.
• The agency has determined that recreational mining, which it has defined in
regulation as suction dredging with a hose no larger than 4 inches in diameter,
does not create a diversion of water and therefore has no effect on water quantities
and flow.

WRD is responsible for granting water rights to various users; they are the body
responsible for permitting and prohibiting various uses of the state.s water. Under the Scenic Waterways Act, WRD (and the Water Resources Commission, which directs the activities of WRD) has multiple responsibilities. Within Scenic Waterways, WRD has the authority to deny a number of uses of water, including dams, impoundments, certain mining operations, and many other activities. Curbing these activities in certain parts of the state was, in fact, one of the primary reasons for establishing the Scenic Waterways System in the first place.

These types of activities are not eligible to receive water rights
on Scenic Waterways. This is true not only on stretches of rivers designated as scenic, but it can also be applicable to areas upstream as well. WRD must make determinations about the likelihood of an upstream activity to significantly affect water quantity. If an activity will ultimately affect the free-flowing character of a Scenic Waterway downstream, then WRD is not supposed to grant a water right for that activity.

WRD will grant new water rights until there is an effect on water quantity, and
there is a standard that has been developed to weigh the effects of new water rights. This standard is known as the Diack flow, which is the amount of water needed in a river to preserve its free-flowing character. The establishment of Diack flows resulted from a lawsuit brought against the state. The case established that WRD is required to limit water usage if such usage will diminish the free flowing character of scenic rivers. WRD has been routinely criticized for not meeting the goals of the Diack flows, and allowing too much water to be allocated.

WRD is also charged under the Scenic Waterways Act to work collaboratively
with other state agencies on actions that involve Scenic Waterways. WRD is to review and concur on management plans for adjacent lands, on land condemnation actions, and on new additions to the Scenic Waterway System, and the agency is given an opportunity to make comments whenever any of these actions take place.


WRD Considers Recreational Mining to Have Minimal Impact; It Does Not Divert or
Take Water

With respect to recreational placer mining, WRD has little to no involvement in
the day-to-day management of these activities. WRD wrote the regulation that defined the parameters of what is meant by the term recreational, and from this definition, DSL and DEQ have established their permitting guidelines and procedures. The definition of recreational as utilizing mechanized or hydraulic equipment, except a motorized surface dredge with a suction hose intake four inches or less in diameter has effectively meant that no recreational equipment is capable of moving enough water to divert or disrupt the free flowing character of any of the states designated Scenic Waterways. These waterways are not small streams and tributaries, but rivers whose flow cannot be significantly altered by a suction dredge with a four-inch hose.

Moreover, water pulled into a suction dredge for mining purposes stays in the river.

There is no taking of the resource out of the river. Thus WRD has determined that there is no diversion or taking of water by recreational placer miners, no water quantity issue at stake, and thus no affect on the free flowing character of the waterway. As far as the responsibility of WRD is concerned, recreational placer mining has de minimus effect. Therefore the agency has no permitting authority or responsibility for this activity, and has expressed no opposition to recreational placer mining in scenic waterways with respect to its responsibilities.

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Biological and Ecological Impacts


The scientific literature reviewed here consists of academic journals and
government reports, including environmental impact statements. The literature covers
biological and ecological impacts that can result from small-scale suction dredging
(studies looking at larger operations were excluded because of their inapplicability to the
issue at hand), and the effects of various mining practices in different types of stream
conditions. This does not imply that all of these impacts always occur in every mining
area.

Different levels of compliance with regulations, varying degrees of land
stewardship, and other contingencies that depend on individual behavior determine
exactly what will occur in a given case. These contingencies, however, are not what the
scientific literature is supposed to reveal -- that is the purpose of monitoring.


Highlights of the scientific literature on environmental affects of dredging include the following:

• There are only a small number of peer-reviewed scientific studies that directly
address the impacts of suction dredge mining. However, there are several
environmental impact statements, government reports and articles (both peerreviewed
and non-peer reviewed) that address the issue, either directly or
indirectly.
• The long-term impacts of suction dredging, tested over large areas and large
populations of fish and wildlife, have yet to be systematically addressed, partly
because of cost and partly because of the limitations in designing and executing
controlled experiments over large times and spaces. Therefore, knowledge
regarding total impacts, especially cumulative impacts, is very limited.
• Scientific literature suggests that there is a high likelihood of some damage from
suction dredging, but it also concludes that with proper regulation and adoption of
certain best management practices, harmful effects can largely, though not
entirely, be mitigated.
• In rivers and streams with high natural fluctuation of waterflows, most evidence
of suction dredging is washed away. Rivers and streams in the region are typified
by high natural, seasonal waterflows. Therefore, most effects of suction dredging
are short term because these high winter flows recruit and redistribute sediment,
recreating natural habitats.


It is important to note that the scientific studies and assessments that have been
done to date, especially peer-reviewed scientific studies, are relatively small in number.
The work that has been done has been generally well-received and well-regarded, but in
spite of the this, scientists in this field are quick to point out that with respect to the
impact of suction dredging, they are much more in a state of ignorance than they are in a
state of knowledge. This is because the studies done to date have looked only at a small
part of what scientists and policymakers may wish to know. They have thus far looked at
impacts over the short-term in localized areas. This in an important area of research, but
it represents only a segment of the possible areas of study. The longer-term impacts over
larger areas and larger fish and wildlife populations have yet to be systematically
addressed, partly because of cost and partly because of the limitations in designing and
executing controlled experiments at large temporal and spatial scales.


Due to the fact that there have not been numerous studies on suction dredging
done in Oregon waterways, and because the characteristics of each river and stream can
be different, this report cannot conclusively state what impacts will be found in Oregon
waterways. However, conditions in streams and rivers throughout much of the
northwestern United States are considered by researchers to be sufficiently similar to
conclude that their results are generally applicable to other waterways.
The literature does suggest, based on studies and assessments done throughout
various parts of the western United States, that there is a high likelihood of some damage
from suction dredging, but it also concludes that with regulation and adoption of certain
best management practices, these harmful effects can largely, though not entirely, be
mitigated.


The scientific literature looks at several aspects of suction dredging and a variety
of potential impacts the practice may have. There are several steps in the process that
may impact the rivers and surrounding areas. They include:

• Access . in order to gain entry to a site, miners may need to drive or walk into
an area without established roads or trails. Also, in order to access a particular
spot in the riverbed to mine, it may be preferable for miners to move large
boulders or logs, or cut or remove vegetation.
• Entrainment . when operating a suction dredge, fish, eggs, fry, or invertebrate
may be sucked into the dredge. This is referred to as entrainment of the
organisms.
• Turbidity . suction dredges, in the process of pulling gravel and sediment up
from the riverbed, may cause a plume that clouds the water, sometimes over a
great distance as water is moved downstream by the current.
• Tailings . during the operation of a dredge, the gravel pile that is formed,
called a tailings pile, can consist of a pile of loose gravel that is not stable
and/or diverts waters from their natural course in the river.
• Sedimentation . fine particles released into the stream by a suction dredge
will not immediately fall to the riverbed and are likely instead to be deposited
elsewhere in the stream in calmer areas where currents are light. This can
potentially cause a buildup of sedimentation in areas where this would
otherwise not have occurred.

Each of these parts of the process may have effects on various aspects of the
ecology and health of the stream and the organisms living in it. The scientific literature
reviewed discusses this and potential cumulative effects of several operations with
respect to fish, invertebrates, riparian habitat, water quality, and cumulative impacts.
Three studies in particular have included a substantial review of other work done
on the question of suction dredge mining. Harvey and Lisle (1998) looked at other peerreviewed
investigations of suction dredging to evaluate the literature and propose
strategies for further study and management of streams and rivers. This study concluded
that the effects of dredging commonly appear to be minor and local, but that effects can
actually vary widely among stream systems and reaches within systems. It is therefore
very important for natural resource managers to take into account the life cycles of fish
and other organisms to tailor regulations to mitigate potentially serious effects. The
authors also noted that there has been relatively little peer-reviewed work done on the
effects of dredging, and the result is that a great deal of uncertainty about its effects
remains, especially long-term effects. Given the current state of knowledge, the authors
suggest that .fisheries managers would be prudent to suspect that dredging is harmful to
aquatic resources.. They further concluded from their review that additional study and
management of streams and rivers was necessary.


A second study was the State of California.s Final Environmental Impact Report
for the Adoption of Regulations for Suction Dredge Mining (California FEIR), issued in
1994 and revised in 1997 to account for some amendments to the regulations. This report
reviewed the relevant literature and discussed several harmful impacts from unregulated
suction dredge mining. The report recommended that proposed regulations would be
sufficient to mitigate these impacts.


A third study, a Draft Environmental Impact Statement (DEIS) completed by the
US Forest Service in 2001, reviewed the relevant literature completed to date for the
purpose of assessing suction dredging activities in the Siskiyou National Forest in
southwest Oregon. The DEIS looked at three alternatives involving varying degrees of
restrictions on suction dredging. It found that the alternative proposed in the draft .
which is generally consistent with the types of regulations currently in effect . would
provide considerable protection to fish, wildlife and habitat through proper regulation,
and that more stringent regulation would offer increased protection.
These and other studies will be addressed below in discussing the potential
impacts on fish, invertebrates, riparian habitat and water quality, as well as cumulative
impacts.


Fish

• Destruction of habitat, entrainment of fish, fry and eggs, loose tailings piles, and
sedimentation can all significantly impact fish populations.
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• Activities of miners must be limited by regulation in order to reduce or eliminate
these impacts.


Suction dredging can affect fish in several ways. Regulations in Oregon allow
mining to occur only in defined in-water work periods, generally in the summer months,
to limit the direct effects of mining on spawning fish, eggs, or embryos. This type of
limitation has a significant effect on preventing harmful impact to fish (California 1994;
USFS DEIS 2001; Harvey and Lisle 1998). In some places, salmonids and nonsalmonids
have spawning and incubation periods that extend into the summer months, so
there is often some overlap and thus potential for impacts (Harvey and Lisle 1998). With
respect to impacts from entrainment, effects are greatest on eggs and fry (Griffith and
Andrews 1981; USFS DEIS 2001; Harvey and Lisle 1998; California 1994).
One study in particular tested this and found that mortality among the eggs of
cutthroat trout sucked through a dredge ranged from 29% to 100% (Griffith and Andrews
1981). The same study also found that sac fry of rainbow trout suffered greater than
80% mortality resulting from entrainment. Mortality rates among sac fry not passed
through a dredge were only 9% (Griffith and Andrews 1981). Other fish that produce
equally small larvae are likely to suffer the same mortality rate from being passed
through a dredge, and eggs from all types of fish would have high mortality rates as well
. resulting not only from the mechanical disturbance, but also predation after passing
through a dredge and other physical/chemical conditions. The State of California.s FEIR
suggested that mortality would be close to 100% for all eggs and fry. Once they were
removed from their protective in-gravel environment they would be available and
attractive food sources that would not be likely to survive for long. By contrast, most
juvenile and adult fish would be likely to avoid being sucked into a dredge, and those that
were would be likely to survive (Harvey and Lisle 1999; Griffith and Andrews 1981;
California 1994).


Suction dredge tailings piles are also believed to have potential impacts on the
survivability of fish eggs, or redds (Harvey and Lisle 1998; USFS DEIS 2001; California,
1994). This may not consistently be the case, as some evidence suggests that gold
dredging in certain streams and rivers increases the availability of spawning gravel by
loosening up compacted gravels (Badali 1988; Hassler et al. 1986). However, impacts
from tailings piles are often considered to be significant. Impacts can result even outside
the work period when miners are permitted to dredge. Dredging does not usually occur
when most fish species of special concern tend to spawn. And many fish spawn in the
spring after dredge tailings from summer and fall mining activities have been moved by
currents resulting from high water flows in the winter. However, dredging during the
summer can affect the reproductive success of fish that spawn in the fall such as chinook
salmon and coho salmon because some of these fish choose tailings as their spawning
habitat, and these piles are typically less stable during high winter flows (Harvey and
Lisle 1999).


The reasons are as follows. During the summer and fall, there is generally low
streamflow. Tailings piles that were created in the summer months are more likely to
retain their original form through the fall, just when chinook and coho are spawning.
Tailings often contain the materials appropriate for redds, although the extent to which
they will be used depends on the amount of suction dredging in an area and the
availability of other suitable sites for spawning. These piles tend to be less stable than
the rest of the riverbed, so when high water flows occur, tailings piles are more likely to
be washed away, resulting in the death of the eggs. Consequently these sites are
considered undesirable spawning grounds (Hassler et al. 1986). The literature suggests
that natural resource managers could reduce or eliminate these potential harmful effects
through regulations that require tailings piles to be redistributed (flattened) in order to
restore the riverbed to conditions more like they were before dredging occurred.
Sedimentation can also pose a problem for fish populations. This can occur
during the period when sediments are suspended in the water and moving downstream .
the problem of turbidity in the water . and after the sediments have resettled in the
riverbed. The impacts of suspended sediment vary with the amount of time sediments are
in the water and the size of the particles. While extremely high levels of sediment can be
lethal, or at least very harmful, it is believed that lethal concentrations of suspended
sediments will rarely be produced by small suction dredges because fish can usually
avoid those concentrations (Newcombe and Jensen 1996; Harvey 1986). Research has
also found that when water is made turbid by dredges, this does not appear to affect the
feeding abilities of many species (Hassler et al. 1986; USFS DEIS 2001). Moreover,
suspended sediments are usually quick to resettle to the riverbed allowing the water to
return to normal levels of turbidity. Dredging is done during low flow periods, usually in
areas where the ability of the stream to carry sediment is low, thus silt is usually
deposited nearby dredging activities. By way of comparison, it has been noted that as
long as multiple dredges are not operating simultaneously, suction dredging cannot
mobilize nearly as much sediment as naturally occurring high water flows (Harvey and
Lisle 1998).


Sediment may also have an impact once it resettles. The concern is that since
suspended sediments tend to fall to the riverbed in areas where waterflow is
comparatively slow, they tend to accumulate in the same places (Keller 1971). When
pools and other habitat are present in these areas, they can be filled in by the sediment,
which has the effect of removing sites fish may normally use (Thomas 1985; Harvey
1986; Harvey et al. 1982). The effects of sediment deposition can also suffocate eggs
and reduce the production of important invertebrates, which are part of the food chain.
However, in streams with larger flushing flows, fish are less likely to be highly sensitive
to dredging, most likely because these same variations in sediment levels occur naturally
(Harvey 1986). The state of California and the US Forest Service concluded that the
effects related to turbidity and sedimentation appear to be measurable at the site level, but
they are localized and temporary in most cases, especially in areas with large natural
fluctuations in stream flow (California 1994; USFS DEIS 2001).


Other impacts can result from movement of boulders and woody debris to permit
access to mine a particular spot. Large items such as these create pools; they also block
the force of the flow, creating feeding and resting areas. Dislodging them can result in
adverse impacts (California 1994, Harvey and Lisle 1998; USFS DEIS 2001). However,
it is also the case that such effects will generally be localized when they do occur. Better
still, they can be mitigated through strict regulation prohibiting the movement of boulders
and woody debris, although the Forest Service has acknowledged that educational efforts
would be required to make sure these efforts were successful (California 1994; Harvey
and Lisle 1998; USFS DEIS 2001).


Some positive impacts of suction dredging have also been noted. In streams
carrying significant amounts of sediment, the riverbed can become embedded and
compacted, providing fish with poor spawning ground. Suction dredging may be able to
break up compacted gravel, particularly on reaches below dams where there are no high
flushing flows (Badali 1988; California 1994). Also, deeper areas left by dredgers may
be occupied by fish once dredging has been completed (Harvey and Lisle 1998;
California 1994).


Invertebrates


• Invertebrate colonies situated in the riverbeds are almost entirely destroyed by
suction dredging.
• The effects of dredging are localized . they do not extend beyond the immediate
area dredged. In addition they are temporary . most invertebrates recolonize
dredged areas within 1-2 months after dredging has occurred.
Invertebrates in streams consist of various types of insects and other organisms
than live in the sediment of the riverbed (these organisms are also referred to as benthic
invertebrates, the benthic zone being the bottom sediment). Several studies have
examined the impacts of suction dredging on invertebrates, investigating whether or not
they are substantially impacted while dredging occurs, and the extent to which benthic
communities recover over time. These studies have shown that the impacts on
invertebrates are significant, destroying invertebrate populations in immediate dredging
area. However, it has also been found that these impacts are generally localized to the
specific dredging area and short-term, with recovery taking place within 1-2 months
(Harvey 1986; Thomas 1985; Hassler et al. 1986; Harvey et al. 1982; EPA 1999).
Griffith and Andrews (1981) found that while fish eggs and fry suffered from
high mortality rates upon passage through a suction dredge, mortality (and injury) rates
for insects were less than 1%. Lewis (1962) found mortality rates of more than 7%,
which is still considered a small number. These results, however, may represent a low
estimate of survival. Observations by both scientific investigators and recreational
miners confirm that fish tend to readily feed on invertebrates dislodged by a suction
dredge, so survival from entrainment may not mean survival through the normal lifecycle
(Thomas 1985; California 1994).


Thomas (1985) found that after dredging, the abundance of insects and other
species at the site was significantly reduced, though these effects were localized. The
numbers of invertebrates downstream were not affected. In addition, recolonization by
insects and other species at the dredge sites were .substantially complete. one month
after dredging. This was determined by comparing numbers of insects at sites that were
dredged with numbers at sites not dredged. Only one type of insect had not fully
recovered in that time. Harvey (1986) looked at sites that had been dredged multiple
times for purposes of his study. He reached similar conclusions to Thomas, noting that
the effects were highly localized, and that insects tended to fully recolonize dredged areas
45 days after dredging took place. He also found that there were no cumulative effects
on invertebrate populations. Similar recolonization took place after each instance of
dredging.


Riparian Habitat


• Riparian habitat can be impacted by three types of practices: by suction dredging
outside permitted areas, such as under the edge of the riverbank and outside the
wet perimeter of the waterway; by cutting or removing vegetation to gain access
to mining sites; and by activities associated with placer mining, such as camping
and blazing trails.


• If these practices are neither prohibited nor mitigated by regulation and good
stewardship, the impacts can last for years before natural processes erase them.
The condition of riparian zones and riverbanks are closely linked to the quality of
habitat for both fish and wildlife. When damage is done to these resources, it can take a
very long time for natural processes to make the needed repairs (Harvey and Lisle 1998;
California 1994). Streambeds and riparian habitat can be most significantly affected in
three ways. The first occurs when suction dredges are used outside the wet perimeter of
the stream, or are used underneath the edge of the stream bank. In these instances miners
can cause significant long-term damage to riparian areas that can last several years.
Suction dredging outside the wet perimeter in the riparian zone impacts habitat used by
many species, large and small. In addition, observations by stakeholders and the
scientific literature both make the point that high water flows remove much of the
evidence that suction dredging has occurred, redistributing gravel, filling or
reestablishing pools. Therefore, areas outside the wet perimeter are not nearly as likely to
be .flushed. by high streamflows when the winter rains arrive, leaving behind a lasting
impact.


When suction dredges are used under the edge of a stream bank, it can undercut
the bank, destabilizing it and sometimes causing it to collapse into the water (Hassler et
al. 1986; McCleneghan and Johnson 1983; California 1994). This destroys habitat for
invertebrates and other organisms that depend on habitat at the water.s edge, and it
removes cover used by some fish species. Undercutting banks also changes the structure
and shape of the waterway, which affects waterflows (Harvey and Lisle 1998; California
1994; Badali 1988). Three assessments done in California found that even though
compliance with regulations was relatively high, some suction dredgers were
undercutting banks in various areas, and the impacts from these violations could be quite
significant (Hassler et al. 1986; Stern, 1988, McCleneghan and Johnson 1983). Though
it is unclear what the compliance rates are throughout Oregon, these results suggest that
significant impacts may be occurring.


Second, cutting or removing vegetation or large woody debris to provide better
access to a particular spot to be mined can have a large impact on the ability of the
riparian zone and streambanks to support fish and wildlife dependent on these areas. The
roots of riparian plants provide banks with stability against flowing water and allow for
non-uniform surfaces along the wet perimeter. Overhanging banks and other irregular
features along the bank can also provide important cover for fish (California 1994; USFS
DEIS 2001; Harvey and Lisle 1998). In addition, vegetation that blocks sunlight from the
river may help maintain cooler water temperatures during the summer, providing better
habitat for salmonid species.


Third, riparian areas can suffer degradation independently of the actual suction
dredging process. Other activities associated with suction dredging such as camping,
blazing trails to dredging sites, and anchoring equipment may impact riparian zones.
Dredge operators camp in areas adjacent to streams and rivers, with campsites sometimes
occupied by families or small groups, and some sites are often camped at for extended
periods (California 1994; USFS DEIS 2001; Harvey and Lisle 1998). In addition, miners
will often camp at sites that are not maintained by land management or resource agencies.
Therefore responsibility for keeping up the site or removing garbage or other traces of
use is left entirely to the miners (Harvey and Lisle 1998). When miners engage in these
practices, which may be a necessary part of getting access to a site or remaining there,
they can cause damage to the riparian zone, which may be home to many species of
plants and animals (McCleneghan and Johnson 1983; Harvey and Lisle 1998; California
1994; USFS DEIS 2001; EPA 1999).


Impacts to riparian areas are thought to be considerable if there is not adequate
control and/or stewardship. In fact, in several instances it was surmised that the
destruction or removal of vegetation and boulders . whether for suction dredge mining or
associated activities . can present a more serious potential environmental threat than the
operation of suction dredges.


Water Quality

• Turbidity is often thought to cause significant impacts to water quality, but this is
not usually the case.
• Suction dredging is more likely to harm water quality from gasoline and oil spills,
from runoff out of adjacent campsites or, to a lesser extent, from mobilization of
heavy metals left by historical mining activities.
• Removal of heavy metals such as lead and mercury by recreational miners can
improve water quality, but may also simply mobilize toxins and reintroduce them
into the water and the riverbed surface.


Water quality is usually unlikely to be significantly affected for a long period of
time by turbidity, which is affected by factors such as the volume of water in a waterway,
rate of waterflow, and the number of dredgers in an area. Turbidity is often thought to be
a significant contributor to diminished water quality. However, larger particles quickly
return to the riverbed, and turbidity caused by fine sediments usually lasts for only a short
time before sediments resettle (Harvey 1986; Badali 1988; USFS DEIS 2001). In Alaska,
the US Geological Survey found on the Fortymile River that turbidity from dredging
causes no significant changes to water quality (USGS 1997). However, the applicability
of assessments on this river to Oregon conditions has been questioned because of
differences in the physical conditions in the river systems.


The water quality of a stream or river may be most significantly affected by
suction dredging as a result of two things in particular. The first of these is that dredges
require gasoline to operate. They also require oil and grease to lubricate parts and keep
the engine in proper working order. Since fuel sources are most likely to be kept within
close range of the dredge, it is likely that there will be periodic spillage of gasoline, oil or
grease into the water and that the impact would be detectable at the site level (USFS
DEIS 2001). With dredges that use 2-stroke engines, this type of spillage into the river is
likely to be even greater. The state of California differs with this conclusion, noting that
while some spillage would be likely to occur, the environmental impacts would not be
significant (California 1994).


Suction dredging can benefit the environment by removing lead and mercury
from waterways (California 1994). As was pointed out previously, Oregon miners
recently turned in ten pounds of mercury. However, suction dredging does not only
result in the removal of these heavy metals. Sometimes it only dislodges them and moves
them around. This mobilization of heavy metals, which reintroduces them into the water
and the riverbed surface, may be toxic to aquatic species (California 1994).


Cumulative Impacts


• Cumulative impacts from multiple dredges operating at once, or from dredging in
the same spot year after year have not been adequately studied.
• This absence represents a significant gap in the literature, and leaves scientists
unsure about the long-term impacts of suction dredging. In the absence of
evidence, scientists advise caution to regulators and land managers.


Cumulative impacts are those that result from either a) multiple dredges operating
in the same area, or b) the same segments of streams and rivers being dredged year after
year. A number of people contacted for this review conveyed their sense that the
cumulative impacts from either of these circumstances may be significant, but also noted
that some cumulative impacts can be subtle and generally undetectable in the short-term.
Harvey and Lisle (1998) have pointed out that no research has been done involving the
impacts of multiple dredges that are closely spaced and operating at the same time, and
that this is likely to be an important consideration, especially in reaches that are dredged
year after year. Badali (1988) suggests differently, however. Using data gleaned from
previous studies not specifically designed to observe cumulative impacts, he notes that
the effects of multiple dredges operating over a relatively limited area of 11 to 15
kilometers are insignificant.


It is also the case that no research has been done looking at the long-term
cumulative impacts of suction dredge mining on areas that are worked every year. (This
may have some relevance for the state of Oregon, where some stretches are mined year
after year with multiple dredges simultaneously. Some mining organizations sponsor
group trips to particular areas, and so more than one dredge may in operation at a time.
This circumstance, however, appears to be more common in areas where an organization
has a mining claim, and less common on specifically designated Scenic Waterways.)
The lack of studies addressing cumulative impacts of multiple dredges and
cumulative impacts over long periods of time represents a significant gap in the literature
about what the impacts and potential impacts suction dredging may be. The unknowns
contributing to such a state are many: the intensity of use on particular waterways,
effects from mining conducted decades ago vs. those resulting from current practices, the
difficulty in tracking whole fish populations, and the difficulty of attributing particular
effects to suction dredging when waterways are subject to multiple uses, to name a few.
Moreover, the cost of turning these elements of ignorance into useful knowledge can be
prohibitive.