Light and Heat in Lakes and Reservoirs - Hydro-Environmental Analysis: Freshwater Environments - page 339

Temperature

Epilimnion

Metalimnion

Hypolimnion

FIGURE 12.15 Idealized winter stratiication.

water does not cool beyond 4°C and lakes and reservoirs remain unstratiied during winter. That is,
rather than mixing twice ( dimictic ), they only mix once a year (one mixing or warm monomictic ).
The 1980 thermal structure of DeGray Lake (reservoir) in Arkansas is used here as an example
of the stratiication cycle of a warm monomictic lake. During January and February, the lake is
unstratiied (completely mixed). During March, stratiication begins (Figure 12.16). The stratiica-
tion continues through the year with increasing temperatures in the epilimnion and the establish-
ment of a strong metalimnion. While mixing across the metalimnion is reduced by the density
gradient, the limited mixing remains suficient to cause some slight seasonal increases in hypo-
limnetic temperatures. Then, in late fall, the surface cools, ultimately resulting in complete mixing
during December through the following spring.
The vertical and temporal temperature variations for DeGray Lake are also illustrated using the
results of the application of the CE-QUAL-W2 two-dimensional model to the reservoir (Martin 1988;

140

130

120

110

100

15 June

15 September

90

15 December

80

15 March

70

60

0 5 10 15

Te mperature (°C)

20 25 30

FIGURE 12.16 Selected 1980 vertical temperature proiles for DeGray Lake, Arkansas.