A numerical model, along with existing field data, is used in an exploratory study of the dynamics of Delaware Bay and River. A three-dimensional hydrodynamic model has been constructed using a finite element discretization in space and a harmonic representation in time. These choices allow for adequate resolution of geometric and flow features but still allow an efficient numerical solution. The nodes in the horizontal are arranged into triangular elements, and the nodes in the vertical are transformed into a terrain-following coordinate system. The nodes in the vertical are arranged logarithmically from the bottom so that the model resolves the bottom boundary layer. This paper focuses on the hydrodynamics rather than the salt dynamics. The model results are sensitive to stratification and to the details of the time dependence of the vertical eddy viscosity, Av. After consideration of several processes, it was found that the stratification effects arise from the tidal straining of the vertical density profile by the velocity. A harmonic expansion of Av indicates that the frictional effects of a time-varying vertical viscosity are more pronounced on the secondary constituents (S2,N2,O1,K1) than the dominant constituent (M2). Thus the ratio of the amplitude of the secondary constituents to the amplitude of the primary constituent provides a sensitive measure for comparison with field data. This comparison places constraints on the variability of Av. ¿ 1997 American Geophysical Union