162 Theories and Techniques of Management 



next, depending upon purposes of water release. If the reservoir is 

 designed to control floods, water will be expelled between floods or prior 

 to anticipated high runoff so that the lake may be partially empty for the 

 storage of excess runoff water. However, if its purpose is to supply water 

 for navigation, the drawdowns occur during the drier parts of the year. 

 In most parts of North America, dry periods correspond to late summer 

 and early fall. However, in some cases, where there is an annual cycle of 

 need for power, the water may be used to generate it, and the drawdown 

 may conform to no schedule, or follow one bearing no relationship to 

 rainfall and runoff in the watershed of the reservoir. 



More experimental work must be done on drawdowns to allow biologists 

 to predict the exact effects of these operations upon fish populations. 

 Nearly all of the experimental work has been on drawdowns made at the 

 end of the summer fishing season. As yet, no one can say whether a draw- 

 down made in mid-summer would be more beneficial than one made in 

 early September. 



Where fishing is an important use for a "waterfowl" lake, there has 

 been severe conflict between fishermen and those concerned with water- 

 fowl, relative to the time that a drawdown is made. Waterfowl managers 

 usually favor a mid-summer drawdown to allow the planting of millet 

 or other quick-maturing grain on exposed mud flats. Fishermen may wish 

 to prolong the summer fishing period as long as possible because, once 

 the lake is drawn down, it may be difficult or impossible to move boats 

 across the exposed mud flats. Moreover, the basin of the lake that is left 

 will probably slope into deep water so gradually that fishing from the 

 bank, once the lake has been drawn down, may be impossible. 



The duck enthusiast will insist that the lake must be lowered in suf- 

 ficient time to absolutely insure a grain crop. The interests of these two 

 groups could be compatible except when the lake bottom is flat and there 

 is too little water left for the fish to survive. In states where spring fishing 

 is permitted, a drawdown in July will have given the fishermen a season 

 of three or four months. They may then well afford to concede to the 

 desires of those who would plant millet or some other duck food crop. 

 Fishermen not only benefit from oxidation of the bottom, but also from 

 the mechanical action of the roots of the grain plants growing in the lake 

 bottom and the lake fertilization resulting from the decay of plant stems 

 and duck excrement in the lake basin. 



Lake Fertilization 



The fertilization of ponds and lakes for increased production of fish 

 has its origin in antiquity, and for centuries it has been common practice 

 in Europe and parts of Asia to fertilize carp ponds. "^^ In the United States 



