Hysteresis and adaptation time scales of salt intrusion and the Turbidity Maximum Zone in a tidal estuary

Salt intrusion and sediment dynamics affect the water quality of estuaries. Multiple factors, such as river discharge, tidal forcing and surge events influence salt intrusion and the location of the Turbidity Maximum Zone (TMZ). As these drivers vary frequently, estuarine systems are in a permanent state of adaptation. This study quantifies the spatio-temporal variability in response to the change in drivers for a typical tidal, funnel-shaped estuary. Using 7 years of monitoring data of discharge, tides, salinity and turbidity, we determined the dynamics of the along-channel displacement of the salt intrusion and TMZ location along the Weser estuary (North Sea, Germany). Both parameters exhibit large lags in their response to changes in discharge. Fast changes such as discharge peaks (Q > 450 m³/s), induce a substantial hysteresis in salt intrusion and TMZ location. The average temporal lag for the system to adapt to a discharge peak is 16 tidal cycles. The influence of tidal range depends on the season (high or low discharge) and the corresponding lag is comparatively small (on average, 3 tidal cycles). Moreover, surge events introduce substantial and random variations of the salt intrusion and TMZ location, without lag. The largest along-channel displacement (18.5 km) is due to discharge, which is 4 times higher than the displacement induced by surge. This study enhances our understanding of the response and adaptation times of estuaries to driver variability and provides insights into how discharge, spring-neap variability of the tidal range, and surge events collectively control salt intrusion and TMZ location.