Lake Melville

Conclusions
First, mass accumulation rates established from
sediment core geochronology show the greatest
accumulation of sediment in western part of the main
Lake Melville basin, on the east side of Goose Bay. In
this location, TOC in surface sediment was three times
greater than at a nearby core site in Goose Bay. This
indicates that sediment is being carried in suspension
eastward in the surface waters of the large Churchill
River plume through Goose Bay Narrows where
currents are fast and flow direction is consistently
eastward (seaward) during both ebb and flood tide.
The effects of strong currents on sediment deposition
(i.e. mixing and resuspension) are reflected in both
the low and homogenised radioisotope profiles and
exceptionally low total organic carbon in surface
sediment on the westward side of Goose Bay Narrows.
Second, although quite removed from the river mouths,
sediment core analysis in the western end of the main
Lake Melville basin revealed a small but significant
increase in terrestrial organic carbon over the last
four decades. It is likely that the Churchill River, as
the strongest source of terrestrial organic carbon to
Lake Melville, contributed to this increase. Specifically,
changes in the flow and drainage area of the Churchill
River caused by the Upper Churchill hydro development
starting in the 1970s could have released terrestrial
organic matter as has been documented for other
hydro developments (Houel et al., 2006), thus likely
increasing the delivery of POC to Lake Melville, at least
for some period until the system readjusted (Newbury
and McCullough, 1984).
Our results show that most of Lake Melville is strongly
influenced by the sediment and organic carbon
delivered by Churchill River inflow, demonstrating a
strong relationship between this river and lake. Algal
production appears to increase in the eastern end of
the lake but otherwise the supply of organic matter
from the river may represent the main support at the
base of the food web. Based on sedimentary records,
there has been a significant increase in supply of
terrestrial organic carbon to Lake Melville post 1970,
which we interpret as most likely reflecting both
change in climate and hydrology of the Churchill River.
5.4. Future changes to sediment and organic
carbon inputs to Lake Melville
With the Churchill River providing by far the most
important sediment source for Lake Melville,
perturbations to the sediment supply from the
Churchill River due to climate change or Muskrat Falls
development will likely impact overall sedimentation,
including the capacity to bury substances (carbon,
contaminants) in Lake Melville. The sites of sediment
accumulation and carbon/contaminant burial will also
be altered, with a decrease in the relative importance
of Goose Bay and increase in western Lake Melville
proper (where other rivers supply sediments). The
newly acquired data on sedimentation rates provide
a baseline for assessing these future impacts. With
changes in the seasonality of river flow having an
influence on salinity, ice transport, and ice volumes
in specific areas of the lake (see Chapter 2), indirect
effects on algal production and hence carbon cycling
and the base of food web may be expected.
The Churchill River is the largest source of sediment
and terrestrial POC to Goose Bay and Lake Melville
delivering 25.2 x 10 8 kg of sediment yr -1 and 18.4 x10 6
kg POC yr -1 to the downstream estuary. Of this, 74%
and 62% of the rivers suspended sediment load and
terrestrial POC is carried beyond Goose Bay and into
Lake Melville proper. To investigate the potential
changes to the sediment and terrestrial organic matter
input from the Churchill River to the downstream
estuary following impoundment at Muskrat Falls,
we applied the median reservoir shoreline erosion
potential of 5.25 m yr -1 per metre of shoreline
reported by Amec (2008) and, following the author’s
assumptions, we assumed a 10 m bank height. Applying
these values to the proposed approximate reservoir
shoreline of 35.5 km and assumed bulk density of
2,600 kg m -3 and assuming half the eroded soil and
organic matter remains trapped in the reservoir and
sand bars downstream, our calculations showed that
the suspended sediment and terrestrial POC load of
the Churchill River to the downstream estuary could
potentially double to 49.5 x 10 8 kg sediment yr -1 and 24.3
x 10 6 kg terrestrial POC yr -1 . It is expected, as observed
in previous impounded systems (see Newbury et al.,
1984), that once the shoreline readjusts to the new
water levels the particulate load of the Churchill River
will decrease. However, this decrease to baseline
conditions could occur over a longer, unknown,
timescale – potentially decades (see AMEC, 2008).
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