The interaction between partially standing or almost progressive surface gravity waves and erodible beds is investigated in a small wave tank under nonbreaking wave conditions. The bed is initially flat and gently sloping, and the incident weakly nonlinear wave is either a regular wave (similar to a Stokes wave), or a regular wave superimposed on a free wave generated by the wave maker (bichromatic or bimodal wave). Accurate measurements of both the bed profile and the wave envelope are performed, showing a permanent coupled evolution of the wave field and of the bed shape. Sandbar formation is observed in the ultimate bed profile which reproduces the spatial modulations of the envelope of the first harmonic of the wave (e.g., the fundamental component). In the case of a pure regular wave, the interbar spacing is equal to half the local surface wavelength which corresponds to a first-order Bragg resonance with the local wavenumber of the first harmonic satisfying the dispersion relation for small waves. In the case of a bimodal wave the preceding case is perturbed by a free wave of the same frequency and similar amplitude as the second harmonic of the regular wave. Then, nonlinear interactions between the first harmonic and the free wave occur, leading to sum and difference frequencies. The difference interaction modifies the amplitude and spatial modulation of the first harmonic. The Bragg resonance mechanism based on the first harmonic in the bar formation is then modified. However, in this case, the bed profile is also the replica of the envelope of the first harmonic of the wave: the perturbation induced in the first harmonic is also present in the bed shape. For partially standing bimodal waves, the Fourier analysis of the bed profile shows a second spatial modulation with the local wave number of the free wave. For almost progressive bimodal waves, a third modulation is found with the local wavenumber of the observed beating between the free wave and the second harmonic component of the wave. These modulations of the envelope of the first harmonic are recovered analytically through a partially standing bimodal wave modeling. This study confirms that the bar formation under nonbreaking wave conditions is controlled by the envelope of the first harmonic of the wave even if perturbed by a free wave. ¿ 2000 American Geophysical Union