www.iisd.org
©2013 The International Institute for Sustainable Development
**Subject to final design and copy edit
largely confined within a counterclockwise current that transports freshwater along the coastline to
Hudson Strait and then to the Labrador Sea and the North Atlantic. Precipitation exceeds evaporation in
the James Bay region, but over most of the complex the evaporation exceeds precipitation.
The formation of seasonal ice cover on the Hudson Bay Complex also results in brine rejection during
freeze-up and the downward convection of denser and more saline waters. The sea-ice melt that adds
freshwater to the sea surface during the spring/summer melt season reinforces the vertical stratification
of the system. Some of this sea-ice melt combines with the outflow from rivers discharging into the
complex. Much of this annual input of freshwater to the system is then transported in the
counterclockwise coastal current that moves northward along the Quebec coast of Hudson Bay before
entering Hudson Strait, along the south coast of Hudson Strait and eventually reaching the Labrador Sea
and the North Atlantic Ocean. St-Laurent, Straneo, Dumais, & Barber (2011) examine what happens to the
freshwater runoff after in enters the Hudson Bay Complex.
Freshwater and the seasonal ice cover play fundamental and defining roles in the Hudson Bay ecosystem.
Stratification, vertical mixing, circulation, salinity, heat fluxes, light penetration, nutrient cycling and
biological productivity are all profoundly influenced by the freshwater in the system regardless of whether
that freshwater is solid in the form of snow, ice or permafrost, or liquid derived from ice melt, rainfall or
runoff from streams and rivers. The seasonal ice cover also makes for a much more continental climate
throughout the region. Less ice cover will make for a less continental climate and an ocean system that is
warmer, probably less stratified, less light and nutrient limiting and, perhaps, more productive.
At the same time, less ice cover will be detrimental to the flora and fauna that are associated with this
habitat. Reductions in the amount and duration of ice cover are recognized as being a threat to marine
mammals such as the polar bears and ringed seals that depend on the land-fast ice cover along the coasts
of the Hudson Bay Complex. Much less attention is usually given to the ice-dependent food web that is
based, in large part, on algae that thrive within the sea ice and on its undersurface. The presence of a
predictable and reliable ice cover is also essential for the aboriginal hunters and trappers who harvest
mammals, fish and waterfowl associated with the sea ice.
The Nelson and La Grande Rivers, which are now regulated for the production of hydroelectricity, are the
two largest rivers entering the Hudson Bay Complex. Both have had their flows augmented by some of
the largest river diversions on the planet and their maximum flows have been shifted from the annual
freshet (May, June and July) to the winter months when the demands for electricity are greatest. Their
combined annual discharges account for about a third of the annual runoff to the system, but during
winter months they contribute almost two thirds of the total runoff. Clearly the shift in the volume and
www.iisd.org
©2013 The International Institute for Sustainable Development
**Subject to final design and copy edit
the annual runoff pattern from these rivers has some effect on coastal salinity and circulation and on the
timing of the annual freshwater pulse that moves along the coast of Hudson Bay and eventually to the
Labrador Sea and North Atlantic. The consequences of these changes in the freshwater budget on
biological productivity and ocean circulation in, and beyond, Hudson Bay are not well understood.
The La Grande River, after receiving major diversions from the Caniapiscau, Eastmain and Rupert rivers,
is now the largest river discharging into the Hudson Bay Complex. Its pattern of discharge has been
fundamentally altered. The average natural monthly flow prior to regulation (1960–1978) was 1,703
m³/second (Hydro-Québec and GENIVAR Group Conseil Inc., 2005) compared to an average monthly flow
of about 4,000 m³/second after adding a flow of 450 m³/second from the diverted Rupert River (Messier,
2002; Hydro-Quebec Production, 2004).
The seasonal changes are much more dramatic. The five lowest-flow months (December to April)
averaged 725 m³/second, which is less than 20 per cent of flows now occurring during these months.
Conversely, prior to regulation, the month of June was the month of peak flow (3,472 m³/second), which
is similar to present mean flows during that month. The spring freshet, a natural feature of unregulated
rivers in the region, is now gone and the maximum discharges now occur in the winter months to meet
the demand for electricity users in southern Quebec and the northeastern United States. While the
pattern of flow of the Nelson River, now the second largest discharge into the Hudson Bay Complex, has
not been altered to the same degree as with the La Grande, it now has maximum discharges in the winter
months.
Granskog, Kuzyk, Azetsu-Scott and MacDonald (2011) provide important new insights into how the runoff
and sea-ice melt inputs of freshwater are transported, stored and mixed within the system. The authors
used oxygen 18 isotope and salinity measurements to differentiate freshwater originating from runoff
from that due to ice melt. The ability to differentiate these different sources of freshwater in samples
from throughout the complex provides a powerful tool for assessing the circulation, mixing, transport and
residence times of water masses throughout the system. It also provides important insights into how the
freshwater budget of the system is likely to respond under climate warming when runoff is expected to
increase (Déry et al., 2011) and sea-ice melt to decline.
Carmack (2000) discusses the complex ways in which the freshwater budget of the Arctic Ocean is so
interwoven with Arctic climate and oceanography:
The study of Arctic climate is truly about the sources, disposition and export of its freshwater
components. As such, understanding the Arctic Ocean’s freshwater budget transcends the