"Lake Life" is a software program designed to introduce the lay person to lacustrian ecology and to the basic concepts of hydraulic management. Having become familiar with the dynamics of the trophic system as well as the mechanisms leading to eutrophication, the user may experiment with managing the reservoir of his choice. Change over time in the principal elements in the ecosystem is calculated by a mathematical model. This paper first presents the successive steps in development of the model: - choice of variables: three plankton groups, fish, nutrients, oxygen ; - representation of complex mechanisms : for example, the vertical structure simulated in two layers ; - transcription into equations. This recapitulation will initiate the reader into the problems of modeling an ecosystem. We shall then analyse a few simulated situations: variations in plankton groups in three situations with increasing trophism, and one example of the impact of turbining on the fish and planktonic populations and on oxygenation of the hypolimnion. The quite realistic behavior of the simulations makes this software an excellent teaching tool.