Thermal Stratification and Loss of Oxygen 31 



out of a pond basin. Some of these are associated with difiFerences in 

 rainfall and evaporation rates. Others involve local situations such as 

 variations in land slope and cover, control of water from a constant source 

 (such as a spring or a flowing well), or the by-passing of excess water 

 w^here the onlv suitable site for a pond is adjacent to a water course too 

 large to be impounded. Solutions to most of these special problems will 

 require the services of a competent hydraulic engineer. 



THERMAL STRATIFICATION AND LOSS OF OXYGEN 



Most artificial ponds and lakes in the United States are thermally 

 stratified during the warmer months. This stratification may develop in 

 earlv March and extend well into November in the South. In the extreme 

 North, summer thermal stratification may begin in late May or early 

 June and end in late August or early September. Most artificial impound- 

 ments, with the exception of the deeper power and water-supply reser- 

 voirs, are eutrophic in character, that is, they contain no oxygen in the 

 colder, deeper waters during the greater part of the period of summer 

 stratification. It is true that early in the season, once the lake has become 

 thermally stratified, there may be oxygen in the lower waters. Gradually, 

 however, the oxygen demand from decay and from respiration of bacteria, 

 plankton, and fishes uses up all of the available oxygen in the lower lake 

 level (hypolimnion) so that this water may be completely devoid of 

 oxygen. 



Sometimes dewatering structures ( spillways ) have their lake-side open- 

 ings near the bottom of the lake, in order to expel oxygen-deficient water 

 from far below the surface. For example, at Ridge Lake, Coles County, 

 Illinois,^ a tower spillway on the inner face of the dam was designed to 

 release water from the bottom of the full lake each time runoff from rain- 

 fall raised its level. Studies of dissolved gases and bottom fauna in this 

 lake indicated that the beneficial effect on it of this disposal of oxygen- 

 deficient water was, at best, very temporary. Even though oxygen was 

 added to the lower levels of the lake each time a substantial rain fell on 

 the watershed, this new oxygen was so rapidly used up that no aerobic 

 bottom organisms had an opportunity to develop. 



Seasonal Thermal Stratification 



A lake or pond is stratified when layers of water at various depths do 

 not and will not mix with one another. For a detailed explanation of all 

 of the ramifications of thermal stratification see Welch, Ruttner, or 

 Hutchison.14' 26, 31 



Rriefly, stratification has its basis in the fact that water shows maximum 

 density (weight) at 4°C (39.2°F), becoming less dense (lighter in 



