On different time scales of suspended matter dynamics in the Weser estuary

Long-term observations in the Weser estuary (Germany) between 1983 and 1997 provide insight into the response of the estuarine turbidity maximum (ETM) under a wide range of conditions. In this estuary the turbidity zone is cl osely tied to the mixing zone, and the positions of the ETM and the mixing zone vary with. runoff. The intratidal suspended particulate matter (SPM) c oncentrations vary due to deposition during slack water periods, subsequent resuspension and depletion of temporarily-formed and spatially-limited dep osits during the following ebb or flood, and subsequent transport by tidal currents. The corresponding time history of SPM concentrations is remarkabl y constant over the years. Spring tide SPM concentrations can be twice the neap tide concentrations or even larger. A hysteresis in SPM levels between the falling and rising spring-neap, cycle is attributed to enhanced resusp ension by the stronger spring tidal currents. There is evidence that the ET M is pushed up-estuary during times of higher mean water levels due to stor ms. During river floods the ETM is flushed towards the outer estuary. If ri ver floods and their decreasing parts occur during times of relatively high mean water levels, the ETM seems to be maintained in the outer estuary. If river floods and their decreasing parts occur during times of relatively l ow mean water levels, the ETM seems to loose inventory and may need up to h alf a year of non-event conditions to gain its former magnitude. During thi s time seasonal effects may be involved. Analyses of storm events and river floods have revealed that the conditions in the seaward boundary region pl ay an equally important role for the SPM dynamics as those arising from the river.