34 Artificial Aquatic Habitats 



Attempts to Upset Thermal Stratification 



Some attempts have been made to upset the thermal stratification of 

 small lakes. ^' ^- For example, 180 hours of pumping of warm surface 

 water into the bottom of a small German lake increased the temperature 

 of the bottom water by 5°C.^ Also, this pumping initiated movements of 

 water within the hypolimnion, thus causing an increase in its thickness 

 as well as a shorter temperature gradient within another stratum, the 

 thermocline. 



In an experiment in a 3.6-acre Michigan lake, water was pumped from 

 the hypolimnion to the surface.^^ This caused a progressive increase in 

 the depth of the epilimnion, a sinking of the thermocline at a nearly 

 constant rate, and a decrease in the thickness of the hypolimnion as the 

 bottom water was displaced. The upper limit of the thermocline was 

 lowered from 13 feet to 25 feet, and the volume of the epilimnion was 

 increased by 49.9 per cent. An attempt was made to follow the movement 

 of the cold bottom water after its release at the surface. Apparently, the 

 cool water became mixed rather thoroughly with surface water within 

 the upper 4 to 5 feet. 



These experiments demonstrate that a large amount of energy is re- 

 quired to modify normal thermal stratification in even a small lake. In 

 lakes of moderate or large size, such a program would be highly im- 

 practical. 



THERMAL STRATIFICATION AND RESERVOIR OUTLETS 



Many large reservoirs are equipped with outlet gates at or near the 

 bottom of the impounding dams. Figure 2.6. Temperatures of water 

 released through these gates range from 4° to 18.3°C (39.2° to 65°F), 

 and may or may not contain sufficient oxygen for fishes. Usually if the 

 water does not have an adequate amount, it become aerated a short 

 distance below the outlet of the dam. In this location, trout are able to 

 survive, and often grow very well, extending their range downstream 

 until the water becomes too warm.-^ Where such a trout fishery has 

 developed, it usually has been necessary to modify the original water- 

 release program designed by the engineers, since to restrict the flow to 

 only a few months of a year is impractical from the standpoint of de- 

 veloping an artificial trout stream. In addition, it is worth noting that the 

 release of cold water alters the bottom faunal pattern from large warm- 

 water species to small cold-water species, such as the insect families, 

 Tendipedidae, Simulidae, and Hydropsychidae, as well as snails and the 

 scud Gammarusr^ 



The tailwater discharge below Dale Hollow Reservoir is an example 

 of a man-made trout stream.-- This tailwater flows for 7.3 miles before it 



