Aperiodic variations of the turbidity maxima of two German coastal plain estuaries

Suspended particulate matter dynamics in estuaries can be split into more or less regular and periodic phenomena dominated by the tide and aperiodic events like river spates which have lasting effects on the suspended matter distribution. The catchment areas of the estuaries of both the River Elbe and R. Weser (Germany) are subject to almost the same meteorological conditions. But the mean freshwater runoff of R. Elbe is about twice the mean R. Weser runoff. In the turbidity maxima of both estuaries, suspended matter dynamics are dominated by the tide most of the year. The turbidity maxima are associated with the low salinity regions, and the locations of both the mixing zones and turbidity maxima depend on runoff. In both estuaries, mixing zone and turbidity maximum react almost immediately and simultaneously on strong increases of runoff. During river floods no turbidity maxima can be observed in the inner estuaries. With decreasing runoff after a river flood the re-establishment of the turbidity maximum lags behind the return of the mixing zone. The restoration of the turbidity maximum to its normal magnitude lasts for months. For the 7 river floods presented here the restoration period varied from 1 to 6 months for the Weser and from 5 to 7 months for the Elbe estuary.