Lake Melville

Figure 2.4. Currents from September
2012 to June 2013 in Year Days
(counted from 1 January 2012) at
51 m depth at M1 inside the sill in
Lake Melville. The U component
represents the horizontal velocity
with direction along the axis of the
lake (i.e. movement in and out of
the lake). The drop in currents from
December 2012 (around day 350) to
late April 2013 (day 500) is because
of the formation of ice.
the surface layer. Surface water exchanges relatively
quickly – in the order of tens of days. We directly
observed deep-water renewal in the lake, finding that
dense water entered over the sill at the end of the year
leading to the exchange of water at the bottom of the
lake. Our results and the observation that no anoxic
events (water without oxygen) have been found in
the deep waters of the lake indicate that deep-water
renewal events are quite common. We postulate that
such renewal events occur over an extended period
of time annually. Overall, the residence time of water
(how long it takes to leave the lake) increases with
depth, and the flushing time of the lake is estimated to
be 192 days based on available observations. We found
that wind-forced and tidal currents in the lake change
significantly in the presence of ice. In the summer,
when there is no ice, the strongest tidal currents are
observed near the surface, in the top 10 m of the water.
In the winter, when ice is present, the tidal currents
increase in strength from the surface to the bottom.
Outside the lake, where open water is common even
in winter, there is little seasonal signal to the tidal
currents.
The currents over the year show the strong influence
of ice in the winter (Figure 2.4). The ice has two
primary effects. The most obvious is the decoupling
of the currents from the wind field. The variability in
the currents that is associated with the wind nearly
disappears in the winter from day 350 to day 500
(several months into 2013). The ice also strongly limits
the surface tidal response inside the lake during the
winter. These two effects mean that the lake is much
‘quieter’ in the winter with weaker currents and less
mixing of water, both in the lake and at the sill.
Influence of Upper Churchill hydroelectric
development on river discharge
In 1971, the Upper Churchill hydroelectric development
began operations on the Churchill River, which flows
into Lake Melville. While findings from Chapter 3
indicate that climate has a much stronger influence
on the ice regime in Lake Melville than Churchill River
discharge, changes in river flow also have the potential
to influence ice cover, though mechanical disruption
and the influence of salinity on the lake. To improve
our understanding of these potential influences,
we conducted model simulations to identify the
response in physical characteristics of Lake Melville
to the changes in river discharge related to the Upper
Churchill hydroelectric development. In particular,
we focused on two major groups of physical factors
that influence ice that may be affected by changes in
river discharge: 1) changes in dynamics and current
velocities in the region adjacent to the Churchill River
mouth, and 2) thermodynamic changes associated with
variations in surface layer salinity. Here we describe
how these two groups of factors impact ice production
and ice transport in the lake.
Evidence from our study demonstrates that the annual
mean river discharge after 1971 is very close to the
annual mean river discharge before 1971, at about 1800
m 3 /s. The major change in river discharge between
these two periods is associated with variations in
the seasonal cycle. After 1971, the river discharge
significantly increases in the winter season (Figure
2.5) when the hydroelectric power facility is working
actively. The large pulse in river discharge in late
spring or early summer is also significantly reduced
after 1971 (Figure 2.5).
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