The nonlinear evolution and the saturation of the collisional Rayleigh-Taylor instability in the presence of a time-dependent ambient density gradient are studied by using a local, two-dimensional nonlinear theory. A set of nonlinear equations that describes coupling of the initial unstable mode with its various harmonics, which are self-consistently generated by the dominant nonlinearity, is derived and then solved to find the saturated amplitudes and the power spectrum of the density fluctuations. In the saturated state the density spectrum is dominated by the fluctuations that are linearly damped but are nonlinearly excited and that have spatial variations only along the direction of the ambient density gradient. It is shown that the time-dependent (increasing) density gradient enhances the saturated amplitude of the dominant modes by a factor of 2 or more, depending on the altitude of the bottomside F layer at sunset. Analysis also shows that the collisional diffusion damping leads to a two-component density fluctuation spectrum with a shallow slope at long scalelengths (>1 km) and a steep slope at shorter scalelengths. The calculated values of the spectral index p, for a power law of the form kx-p, are p≅2.0 for the shallow slope and p≅3.0 for the steep slope, and they are close to the values determined from the one-dimensional measurements.