A turbidity maximum is consistently observed in the upper reaches of the Tamar estuary at high water. Neap tide suspended particulate matter (SPM) concentrations are typically 20 ppm at high water; spring tide concentrations are typically an order of magnitude greater. The location of the turbidity maximum is usually associated with the freshwater-saltwater interface but occasionally occurs further up-estuary. At neap tides the maximum is less defined and can be masked by freshwater SPM concentrations. The spring tide maximum is generally small during winter periods, when it is located down-estuary, and reaches its largest concentrations during summer when it is close to the head. About 65% of the variance in the magnitude of the maximum at spring tides can be explained in terms of its distance from the head. The position of the maximum is determined mainly by freshwater runoff with a logarithmic regression explaining about 80% of the variance. Only 30–35% of the variance in the turbidity maximum magnitude and location at neap tides can be explained in terms of tidal range and runoff. A tidal resuspension model is considered which ignores density effects but which has a spatially independent, runoff dependent (but otherwise time independent) erodibility constant as a single “free” parameter. The model provides a reasonable description of the magnitude and location of the turbidity maximum in the Tamar, both at spring and neap tides, despite the fact that the maximum usually occurs near the freshwater-saltwater interface (which in itself would strongly suggest that gravitational circulation and intratidal stability effects have a strong influence on the location of the maximum). It is possible that all three mechanisms act together to produce a much narrower maximum than we have simulated, with resuspension being enhanced in the strongly mixed fresh water behind the interface, and tidal pumping and gravitational circulation accumulating sediment close to the interface.
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