A Comparative Study of Different Types of Thermal Stratification in Lakes and Their Influence on the Formation of Marl

Abstract

The results of an, investigation of thermal stratification in two of the Great Lakes are compared with the thermal stratification which characterizes small lakes in the Ottawa Valley. It is shown that the warm zone or surface water commonly found in the Great Lakes during the summer has several times the thickness (five times in one comparison) of the similar surface zone of a small, relatively shallow lake. While the bottom temperatures of the large, deep lake and small, shallow lake may be essentially the same, the surface or epilimnion zone in the large lake is a few degrees lower than in the small lake, elevation and latitude being about the same. With increasing elevation in a mountain region thermal zoning becomes less distinct and may disappear entirely at the end of a valley glacier. The relatively low temperature of the epilimnion zone in the Great Lakes and in high mountain lakes prevents by producing conditions unfavorable to marl formation the development of marl beds in lakes of great size, or in small lakes above 5,000 feet in Southern Canada. Latitude, by controlling the temperature of lakes, sets a northern limit to the formation of marl beds, which is probably not far from the northern limit of forests in North America.