We have used a mode-coupling hypothesis to study the nonlinear evolution of E region irregularities at high latitudes. Conservation of energy and the identification of two distinct time scales for the problem at hand has allowed us to obtain an expression in the fluid regime for the mean frequency and the spectral width of different types of echoes observed by coherent radars. In this particular paper we have applied our results to a few simple cases, namely, to situations that are free of large-scale gradients and for which aspect angles are close to zero. Even though anomalous diffusion effects were also neglected, our theory nevertheless predicts that in the absence of density gradients, strongly driven Farley-Buneman waves should normally saturate at a mean speed between 70% and 100% of the ion-acoustic speed of the medium. The theory also predicts that zero-frequency type 2 waves in strongly turbulent situations should have a frequency width, which when translated to a Doppler width, should be approximately equal to the ion acoustic speed of the medium. Those results are a direct consequence of assuming that mode coupling is responsible for the saturation of all linearly unstable waves in the E-region plasma. A companion paper will consider the modifications introduced by large-scale density gradients. ¿ American Geophysical Union 1993