the least change and where fluctuations due to
They found that a common source of turbid-
other causes would be more significant.
ity was the clay shorelines common along the river
A study by Gleason et al. (n.d.) examined sedi-
and that wind-driven waves of 6 in. or more in
mentation rates on the St. Marys River by plac-
height could generate a high level of turbidity
ing nine samplers at three locations: one in a chan-
extending from the shore to the navigation chan-
nel closed to winter navigation and two along
nel. Under those conditions, no effect of vessel
active channels. Sedimentation data were collected
passage could be discerned. Several of the sites
between 7 and 27 March. The samplers were left
used to monitor other vessel effects examined in
in place during breakup, but five were lost and
their study were sufficiently turbid throughout
only two retained usable samples. Their analysis
all field periods that it was impossible to see the
of the data at one site indicated an increase in
riverbed. Their major findings were:
sedimentation with vessel traffic, although rates
The nearshore zones have more turbidity
were low (averaging 1.3 mg/day in their 18.5-
than the navigation channel, both with an
cm2 sampler area). A second site showed no rela-
ice cover and no vessel traffic and with open-
tion between sedimentation rates and vessel pas-
water and vessel passages.
sage, perhaps due to high natural turbidity. The
Navigation channel turbidity was less in
third site, which had the highest sedimentation
March than in May or June.
average at 11.1 mg/day, showed a correlation with
In general, near-shore turbidity decreased
vessel passage. Further, sedimentation rates were
with the removal of the ice cover.
found to decrease with increasing distance from
The turbidity in offshore areas of Lake
the channel.
Munuscong (but away from the channel) was
They concluded that winter navigation can in-
least with an ice cover and most in June.
crease sedimentation rates over ambient condi-
Sites on Lake Nicolet showed a decrease in
tions, but that natural sedimentation during spring
turbidity after ice-out.
breakup also causes sedimentation rates equal-
The Charlotte River is a major contributor
ing or exceeding those due to navigation. They
of sediments causing turbidity.
cited sedimentation rates on a channel with sig-
Finally, vessel-induced turbidity was observed to
nificant navigation being 50 times greater than
be slight near the channel and highest near the
at their control site, but the large difference in
shore, indicating that ship waves and drawdown
site conditions (and thus natural sedimentation
and surge were generally more significant than
rates) makes the comparison of questionable va-
propeller wash.
lidity. The authors did not find significant spawn-
Poe et al. (1980) also measured light extinc-
ing areas in the St. Marys, nor did they demon-
tion on the St. Marys River during the winter of
strate or discuss the effects of sedimentation rates
1978-79 during a period with winter navigation.
on coregonine eggs.
They chose two river areas for study, and they
Hodek et al. (1986) conducted a field investi-
selected what they considered to be high- and low-
gation of ship-generated turbidity on the St. Marys
impact data collection sites within each of these
River. They provided the results of 95 measure-
areas based on a perceived difference in the po-
ments of turbidity and 85 light extinction pro-
tential for vessel passage effects. The basis for
files under both ambient and ship-influenced con-
determining the level of vessel impact potential
ditions. Unfortunately there were no vessel pas-
is not clear, nor are differences in site conditions
sages during sampling periods with an ice cover.
apart from vessel effects explained.
Ambient turbidities during open-water conditions
All measurements were collected during or im-
were typically in the range of 530 JTU, although
mediately following vessel passage except for
numerous points were higher and the maximum
those made during March. Observations in March
reading was 380. Measurements during open-wa-
had no vessel passages and were considered as a
ter vessel passages typically ranged from 6 to 30
"control" condition. All measurements were taken
JTU, with a maximum of 53. This information has
through the ice, but by the April field period the
been incorporated into the database of their nu-
ice cover had become fragmented. They found
merical model of the physical effects of vessel
that light penetration was generally lower in Feb-
passage, which primarily deals with sediment
ruary than in March or April and that light pen-
transport and shoreline erosion potential. How-
etration was greater at their low-impact sites than
ever, it was not directly incorporated into the
at the high ones.
numerical calculation scheme.
Based on records of ship passage they felt vessel
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