The subject of water dynamics in retention lakes is not a well known area of our knowledge. When it comes to model the hydrodynamics of dam lakes severe difficulties appear. This is not because the governing equations are not known. In fact there is a good choice among the formulas to use. The problem is that the boundary conditions for those equations are not well known. This applies to both spatial and time-related data. Exact values of: bed elevation and roughness distribution, input and output discharges, rainfall and evaporation values, wind speed and direction are known only as...