ABSTRACT
The St. Lawrence estuary in eastern Canada (Figure l ) is the region where the waters of the Atlantic Ocean and those of the Great Lakes CO-mingle. It is 500 km from the Great Lakes and nearly 1000 from the ocean and is a middle ground generally characterized by waters abundant in life. Apart from its three-dimensional characteristics, large size, and topographic features, the St. Lawrence estuary provides a unique case to study from two other standpoints. First, the formation of ice in the winter months affects the wind-induced circulation and mixing. Ice formation starts in December, and there is almost a complete ice cover in the estuary from about January to March.' In addition to the effect of reducing wind-induced mixing and circulation, the ice cover provides another boundary layer which increases the effects of friction and the accompanying generation of turbulent mixing processes. Furthermore, the freezing and melting of sea ice must be considered, as the formation of ice removes freshwater from the water column and releases it upon melting. Although the total annual discharge from this process is zero if the advection of ice can be neglected, the equivalent discharge is at times significant. Using data given by Forrester and Vandall,2 Budgen et al.3 estimated the discharge implied by ice freezing as - 8 X 103 m3/sec in midFebruary and of the same magnitude, but positive, in mid-April. As will be seen later, this is a significant effect compared to the river discharge from the St. Lawrence and major tributaries.
