If the impact of longitudinal density gradient on the hydrodynamics of estuaries is negligible, then in water quality simulations, the hydrodynamics can be decoupled from the mass transport which would lead to substantial savings in computational time and storage requirements. It is thus important to be able to assess this impact beforehand.

The impact of longitudinal density gradients is evaluated analytically in the context of a one-dimensional wide rectangular channel taking into account the influence of bed slope, bottom friction, type of boundary condition and transient effects. The analysis predicts actual errors that would arise in numerical computations from near exactly in the best cases to within a factor of 6 in the worst cases. It should thus be useful for at least an order of magnitude prediction of the errors that would arise from the neglect of the effect of longitudinal density gradient on estuarine hydrodynamics.