The natural thermal cycle of a stratified water body used for power-plant cooling will be disturbed both by heat addition and the mixing effect of withdrawal and return. A perturbation analysis for these effects is made with a model based on the assumption that a Richardson number is constant at the base of any stratified layer. On a further assumption about the profiles of wind-driven return currents, constant heat flux from that layer is inferred. This heat flux, and the diffusion coefficient at the thermocline, are the critical parameters of the simple one-dimensional line-segment model, and are chosen to give good imitation of the known natural cycle of Cayuga Lake. The model is then perturbed in terms of both heat flux and diffusion to give power-plant impact for that lake. Both transient and final cycle changes of summer and winter temperatures and stratification and overturn are calculated. It is shown that the heat and diffusion effects are comparable, and that the latter may be dominant if the discharge is diluted to meet a thermal standard. Certain implications as to strategy of water use are developed.

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