Thick layers of fluid mud occur in strong tidal flows over the inner portion of the Amazon continental shelf in regions of strong salinity fronts associated with the plume discharged from the Amazon River. Detailed shipboard profile measurements obtained in this region during A Multidisciplinary Amazon Shelf Sediment Study (AMASSEDS) provide an unprecedented opportunity to examine the structure and dynamics of fluid muds under natural conditions. The analysis focuses on flows in which the motion is fully turbulent and suspended sediment dominates the stratification. Under these conditions a comparison of measurements and one-dimensional model calculations indicates that vertical transport is controlled by suppression of turbulent mixing at gradient Richardson numbers near 1/4. This constraint produces a distinctive vertical structure and leads to an upper bound on the total amount of suspended sediment that may be carried in a turbulent suspension by a tidal flow. ¿ American Geophysical Union 1994