Turbidity was generally lower during periods
ing April, and the amount of sedimentation be-
of winter ice than for open-water conditions. Win-
tween deployment of each sampler could be es-
ter means were between 0.5 and 2.3 NTU, while
timated incrementally. Most stations indicated a
summer means ranged from 1.3 to 45.5 NTU with
trend of increasing sedimentation rates with time
from January through March, but no explanations
no vessel traffic.* During open water, ambient
were given. Average rates during the entire sam-
turbidities generally decreased from shallow zones
pling period ranged from 94 to 483 mg/daym2,
out to the navigation channel and increased with
distance downriver due to tributary inputs and
with a median of 310 and a maximum measure-
broad expanses of shallow water subject to wind-
ment of 850 at one site during 8 March to 9 April
driven wave action. Turbidity measurements dur-
1985.
ing open-water vessel passages showed no val-
Ship passages were monitored at two sites, one
ues in excess of 11.8 NTU at any of their sites,
on each river. The Detroit River site had 24 pas-
which is within the range of natural variation.
sages sampled, equally split between field trips
Winter sedimentation rates without the presence
in August 1983 and April, August and December
of navigation ranged from 53 to 2400 mg/daym2,
1984. The St. Clair River site had 18 passages
with a median value of about 962. The mean par-
sampled, evenly split between the three 1984 field
ticle size from all samples was estimated to be
trips. April and December were considered win-
between 50 and 60 m.
ter, while the August trips constituted summer.
Sletten (1986) conducted a two-year study of
Water samples were collected at intervals following
the water quality effects of extended season op-
the passage of the bow for periods of 30 or 60 min-
erations on the SCDRS. Included were documen-
utes, providing a time record of water quality varia-
tation of background water quality, sedimenta-
tions. Although levels of turbidity and suspend-
ed solids were found to vary following vessel pas-
tion rate data and water quality variations with
sage, all maximum values recorded were signifi-
time during vessel passages. The background
cantly less than natural variations in background
water quality information was primarily summa-
levels. No significant correlations between ship size,
rized from existing databases supplemented by
speed or season of passage, and measured changes
a limited amount of data collection. The primary
in water quality parameters were detected.
emphasis in the analysis of these data was to lo-
Possible reasons cited for the lack of any cor-
cate extreme values of total suspended solids and
relations were that none exist, that correlations
turbidity for comparison with vessel passage
exist but are too complex for analysis, and that
events. Other variables examined were pH, tem-
the samplers were not located properly. However,
perature and dissolved oxygen. Average turbid-
Sletten used linear regression with single ship vari-
ities were found to vary from 8.7 JTU in the win-
ables (draft, displacement or speed) to examine
ter to 7.3 in the summer, but temporal variations
within a season were large. Mean values of sus-
correlation. Correlation on this basis would re-
pended solids, pH and dissolved oxygen did not
quire equal effects for large and small ships if they
vary significantly between seasons.
traveled at the same speed, or equal effects for a
Baseline sedimentation rate data were also col-
single ship traveling at different speeds. A lumped
lected by Sletten (1986) at two sites on each river
parameter reflecting both ship speed and size
in shallow, off-channel areas. Samplers were typi-
would be more appropriate. Further, the data show
cally placed at each site monthly from December
that the elapsed time from ship passage to the
or January through March in both years. All sam-
maximum recorded parameter values ranged as
plers were collected simultaneously the follow-
high as 60 minutes, which was the maximum
period of sample collection. While vessel passage
effects can persist for a relatively long time, it is
* In the discussions that follow, turbidities are expressed
curious that maximum values were often found
in both Jackson Turbidity Units (JTU) and Nephelom-
as much as an hour after the event, probably in-
etric Turbidity Units (NTU) in order to retain the units
dicating other causes. Hodek et al. (1986) found
employed by the authors of the reports under review.
that spatial variations in turbidity were large, even
Although these units are roughly equivalent, turbid-
under ambient conditions on the St. Marys River,
ity readings are influenced to some degree by the mea-
and that the maximum levels of ship-generated
surement technique and the characteristics of the ma-
turbidity were near the shoreline. Sletten's sam-
terial in suspension. For the purposes of this report,
pling was conducted at the edge of the naviga-
however, it should be possible to consider these units
tion channel, where Hodek's observations showed
equivalent.
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