Runoff and tidal influences on the estuarine turbidity maximum of a highly turbid system: The upper Humber and Ouse Estuary, UK

Results are presented from longitudinal and vertical measurements of salinity and turbidity made at spring and neap tides throughout the length of the upper Humber and Ouse Estuary, UK, during a 1-year period that experienced strong freshwater runoff during winter and a long period of very low runoff during summer. These data were complemented by tidal-cycle profiling measurements at a site in the lower Ouse. Despite the strong suspended particulate matter (SPM) stratification, salinity was fairly well mixed vertically except in the presence of high concentration, near-bed muddy layers. The data consistently exhibited a strong estuarine turbidity maximum (ETM) in the low salinity reaches of the estuary. The geographical positions of the freshwater–saltwater interface (FSI), XS, and the up-estuary intrusion of the ETM (the ETM nose, XP) measured from the tidal limit, exhibited a pronounced seasonal cycle. A multiple regression for XP in terms of the monthly-averaged runoff, Q31 (averaged over 31 days preceding the survey) and the tidal range for the surveyed tide, TR, ‘explained’ 97% of the variance in XP. The ETM nose was located in the lower estuary during high runoff winter conditions, more than 50 km from the tidal limit, and in the upper estuary during very low runoff summer conditions, within 2 km of the tidal limit. According to the regression, the ETM nose was shifted up-estuary by 15 km between mean neap tides and mean spring tides. The location of the FSI also depended strongly on runoff. The regression for XS in terms of TR and Q31 explained 88% of the variance. The FSI was located in the lower estuary during winter conditions and in the upper estuary during summer conditions and according to the regression there was an up-estuary displacement of 6 km between mean neap tides and mean spring tides. The separation between the ETM nose and FSI, (XP − XS), tended to be positive (ETM nose down-estuary of the FSI) when the nose was located more than about 55 km down-estuary from the tidal limit, but was otherwise negative (ETM nose up-estuary of the FSI). When the nose was located within about 30 km of the tidal limit, it tended also to be about 10–30 km up-estuary of the FSI. A multiple regression of (XP − XS) in terms of XP and TR explained 78% of the variance. The magnitude of the maximum, depth-averaged SPM concentration within the ETM, P, was most closely related to the location of the ETM nose. The logarithm of P decreased approximately linearly with XP and P was reduced from 35 g l− 1 during summer, when XP was less than 2 km, to 3.5 g l− 1 when XP was 58 km. A regression explained 94% of the variance. Tidal-cycle profiling illustrated the seasonal ETM movements via the presence of very high turbidity waters in the lower estuary at high water during winter and spring, at low water during summer and early autumn and at mid-ebb through to mid-flood in the late autumn. This seasonal movement of the ETM also was accompanied by a seasonal variability in sand-sized subtidal and intertidal bed-sediment distributions within the upper Humber and Ouse Estuary in which very fine sand was transported down-estuary over the high runoff winter period.