Physiography of Lakes and Reservoirs - Freshwater Ecology: Concepts and Environmental Applications - page 109

ponds and lakes, features such as hills or trees can prevent the wind from
making very large waves. For example, deforestation can lead to a deeper
epilimnion (by about 2 m in some small Canadian lakes) because of increased
mixing during spring warming (France, 1997a). Even though the processes of
surface wave formation are the most apparent to human observers, the wind
causes other water movements in lakes that can also be important.
As wind moves water at the surface of a lake forward, it must be re-
placed by water from below. This process leads to spiral circulation pat-
terns called Langmuir circulation cells (Fig. 6.15). The spiraling water
moves in alternating directions, leading to lines of downwelling water al-
ternating with lines of upwelling water. These lines form along the direc-
tion of wind. Floating materials aggregate at the water surface along the
downwelling lines and form streaks in the same direction as the wind is
blowing. These circulation cells are several meters wide.
In addition to the smaller scale waves and Langmuir cells, movement
of water can also occur within the whole lake's volume. When a sustained
wind occurs, it causes water to pile up on the downwind side of the lake,
and when the wind suddenly ceases the surface of the lake can rock. This
rocking of a lake's entire surface is called a seiche .
An interesting phenomenon occurs in stratified lakes that are subjected
to a sustained unidirectional wind. The force of the wind causes the water in
the epilimnion to move across the lake to the downwind side (Fig. 6.16), and
the depth of the epilimnion is greater downwind than upwind. Under extreme
winds, the hypolimnion can come to the surface on the upwind side. When
the wind ceases, the less dense water of the epilimnion moves back across the

Top view

Floating streaks

Water surface

Side view

FIGURE 6.15

Langmuir circulation cells on a lake (top view and side view).