Seasonality of the turbidity maximum in the Humber-Ouse Estuary, UK

A 3-year campaign of field measurements in the Humber-Ouse Estuary was undertaken as part of the UK's Land Ocean Interaction Study. The measurements identified a persistent, high concentration turbidity maximum (TM) in the upper estuary that was generally associated with the freshwater-saltwater interface (FSI) region. Seasonal variability in the locations of the FSI and TM was pronounced and consistent from year to year. The TM was located more than 90 km down-estuary from the Ouse's tidal limit following freshwater spates during February 1995, whereas it was located less than 30 km down-estuary following the drought of summer 1995. Taken over the whole record, the mean positions of the FSI and TM were 47 and 52 km from the tidal limit, respectively, and their standard deviations were 13 and 20 km, respectively. There was a lag between the movements of the FSI and the TM. Maximum and minimum locations of the TM, relative to the tidal limit, occurred about one month after those for the FSI. During winter months the TM was located further down-estuary than the FSI, and was relatively weak, whereas during summer and early autumn the reverse was true and the TM was much stronger and located much closer to the tidal limit. Statistica- l relationships are derived that describe the FSI and TM in terms of freshwater inflows, tidal range and location. These results indicate that tidal 'pumping' effects are important to the formation and migration of the TM, particularly during low inflow conditions, and that sediment 'trapping' due to gravitational circulation may also play a significant role, especially during high, freshwater inflow conditions.