Water above 39 degrees sinks when it gets colder. Water colder than that stays on the surface.
Getting Ready to Freeze
Obviously a lake has to get cool enough to freeze before we will have anything to play on. This starts in the fall as water is cooled at the surface. This denser surface water sinks. Water has the unusual property that it reaches its maximum density at 39 degrees. The lake cools by density and wind driven convection until it gets to 39° from top to bottom. This shuts down density driven convection however wind driven convection continues at full force and can mix the whole lake as everything is the same density. This is called 'Fall Turnover'.
Wind driven mixing continues to cool the whole lake until the lake catches. It typically takes a cold, very light wind night to get the first ice sheet to form. In big lakes like Lake George or Lake Champlain the wind keeps the ice at bay until it is within about 2 degrees of freezing. Big lakes also often come in in stages with the deepest, most wind exposed places coming in last.
Once an ice sheet is established the wind mixing stops and the lake enters ‘winter stagnation’. The density gradient of the water unter the ice is stable (lower density, colder, water is on top) and the wind can no longer drag the water around. Click here for a nicely prepared webpage on lake temperature cycles by the Geology Department at Grand Valley State University.
Actual lake temperature profiles typically have 32 degree water right at the bottom of the ice which increases by 2 to 5 degrees over the next foot or two and then stays relatively constant at greater depth. We call this the 'transition layer'. Most rivers have enough flow mixing to prevent a transition layer from forming. This results in water temperatures of about 32.1 degrees throughout the depth of the river and also accounts for the different behavior of river ice.
There are often slow (0.1-0.2 ft/second) currents under the ice but, for the most part, they do not appear disturb the transision layer. Exceptions include where the currents pass over reefs or through narrow places where flow is concentrated. Forced convection situations like gas holes, dock bubblers and off edge winds also bring warm water to the surface (see Reef Holes Gas Holes Off Edge Breakup).
Click Here for an article on lake turn over on Lake Champlain by Nancy Bazilchuk.
Bob
