PART VIII — AQUATIC ECOSYSTEMS 



Figure VIII— 13 — THERMAL INFLUENCE OF 

 ELECTRIC POWER GENERATION ON LAKE MICHIGAN 



The table shows estimates of the effect of heat input into Lake Michigan due to 

 waste heat from the generation of electric power. These effects are indicated in 

 terms of an increase in lake temperature and an increase in evaporation. 



If we consider only the inshore 

 strip of water (of depth less than 10 

 meters, or 33 feet), which covers 

 8 percent of the lake area, the picture 

 looks different. In that case, the 

 daily total input of heat from power 

 stations for every day of the year 

 by 1990 would be about 10 percent 

 of the sun's input on a summer day. 



Temperature Rise — Another way 

 of looking at the matter is to con- 

 sider the temperature rise of the 

 whole or part of the lake attributable 

 to power-station inputs. This is a 

 much more complicated and uncer- 

 tain calculation, because of lack of 

 knowledge of the rate of dispersion 

 and of how long the heat stays in the 

 lake before it is lost to the atmos- 

 phere, or to space by back-radiation, 

 or to increased evaporation. This re- 

 tention time is a statistical estimate 



in any case; it is certainly greater 

 than one day and probably less than 

 30, so a guess at 10 days seems not 

 unreasonable. With that guess we 

 find, again, that the effect on the 

 whole volume of the lake is negligible 

 but that the effect on an inshore 

 water strip is appreciable. For ex- 

 ample, the temperature rise of the 

 inshore strip, based on these assump- 

 tions (10-day storage and complete 

 mixing into this inshore volume), 

 would amount to 0.7 centigrade — 

 i.e., a little over 1° Farenheit by 1990. 

 These estimates do not, of course, 

 take into account any major engi- 

 neering changes or advances in de- 

 sign leading to better thermal effi- 

 ciency. 



The significant conclusion from 

 this is that, because the heat input 

 takes place at a number of point 



sources, there will be measurable 

 temperature rises locally but the aver- 

 age effect on the whole lake will not 

 be substantial. It is with local effects, 

 then, that we must be concerned. 



Natural Phenomena — The natural 

 temperature regime of the coastal 

 water is complex. In summer, there 

 is sporadic upwelling of cold bottom 

 water, depending on the stress of the 

 wind over the whole lake, coupled 

 with the effects of the earth's rota- 

 tion. The temperature at near shore 

 intakes (for example municipal water 

 plants) can sometimes change by 

 many degrees in an hour. 



Another phenomenon that adds to 

 the complexity of coastal circulation, 

 and which is not this time dependent 

 on changes in the wind, is the so- 

 called thermal bar. This is most 

 marked in spring, when the shallow 

 water near shore is warming up to 

 temperatures above that of maximum 

 density (4"C), while the offshore 

 waters remain at their winter tem- 

 perature below 4° centigrade. Where 

 the warmer inshore and colder off- 

 shore waters mix, a water mass is 

 formed close to the temperature of 

 maximum density. This mixture is 

 heavier than the original inshore and 

 offshore water masses from which it 

 was formed, and it therefore sinks. 

 This continually sinking water mass 

 (a convergence) forms a temporary 

 barrier to horizontal mixing between 

 inshore and offshore waters. At the 

 same time, the convergence is a rather 

 efficient way of carrying water (and, 

 therefore, heat) from the surface into 

 the deeper regions of the lake. 



As the spring heating continues, 

 the thermal bar migrates further and 

 further offshore until, usually some 

 time in June, the summer thermal 

 stratification is established right 

 across the lake. At times when the 

 thermal bar is strongly established, 

 water may be trapped inshore for 

 several days or weeks. The effect of 

 thermal discharges into that trapped 

 water mass is a matter for conjecture. 

 But it seems likely that situations 



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