OBSERVATIONS in estuaries have reported the existence of ephemeral deposits of semi-fluid mud1,2 extending several metres from the bottom. These muds differ from ‘mud’ as normally understood in that although they form definite boundaries with the overlaying water mass they have a lower solid content and settle, if at all, only very slowly. They are also reported to have only 4–400 mg ml−1 suspended solids compared with an average of 1,300 mg ml−1 for bottom muds3. Fluid muds can be detected only by high frequency (30 kHz) echo sounding techniques and have been found to vanish at certain periods of the tide cycle.