SUMMARY
Field studies using acoustics, optics and current meters were undertaken of fluid mud entrainment in the extremely turbid Jiaojiang River estuary, China. The lutocline behaved as a density interface on which rode high-frequency internal waves. These waves were locally generated by the interaction of shear and buoyancy. Their breaking accounted for vertical mixing, however mixing events proceeded mainly upward, rather than downward, as a reason of the sediment-induced collapse of the turbulence in the lower layer. This asymmetry calls for a new parameterization of vertical mixing in very turbid estuaries and coastal waters.