The hydrodynamic equations are reviewed and simplified for use in a study of sinking cooling water plumes. Two examples of observed sinking plumes are shown. A numerical model is employed to study the magnitude of different terms in the equations and the effect of varying turbulent diffusion coefficients. The model is a two dimensional one (one horizontal and one vertical dimension).It is concluded that the sinking phenomena is very sensitive to changes in the vertical diffusion coefficient and it is recomended that future three dimensional studies are carried out in a model containing a reliable computation of the turbulent fluxes .