Pulses of 29, 50, 82, 120, 192, and 326 kHz were transmitted at a shallow-water site uniformly 12 m in depth. From this data set amplitude fluctuation as a function of source--receiver separation distance was obtained. The seafloor at the study site consisted of soft muds so that the direct path and a surface bounce path were the primary components of the received signal. In order to estimate turbulence parameters, direct path arrivals were analyzed at ranges where they were separated in time from the surface bounce arrivals (for separation distances less than 130 m.) Analysis of the log-amplitude variance of direct arrivals shows agreement with weak scattering theory using a Kolmogorov turbulence model and yields an average structure constant C[inf n][sup 2](approximately equal to)8x10[sup -9]. The estimated values for the structure constant were used to predict the contribution of the refractive index fluctuations to total amplitude variability for the entire range of source--receiver separation distance. It was found that for this environment amplitude variability is dominated by sea-surface roughness for source--receiver separation distances out to ~200--250 m and by refractive index fluctuations for greater separation distances.