Remotely sensed observations of the turbidity maximum in the highly turbidHumber estuary, UK

The Humber Estuary, UK, divides into the Ouse and Trent estuaries at the so -called Apex within its upper reaches. Remotely sensed Compact Airborne Spe ctrographic Imager (CASI) images and boat measurements were used to observe a strong turbidity maximum in the upper Humber and Ouse during a spring ti de in November 1995. Surface suspended particulate matter (SPM) concentrati ons during the late ebb, as estimated from the CASI data, increased from ap proximately 6 to 13 g l(-1) moving up-estuary into the Ouse. Greater SPM co ncentrations (similar to 10 g l(-1)) were evident in the deeper channels of the Ouse, compared with shallower areas, possibly due to faster ebb curren ts there and differential down-estuary advection of the turbidity maximum. Ribbons, or streaks, of lower SPM and slightly cooler waters were observed. It appears that slightly cooler and lower turbidity waters from the conflu ent Trent estuary remained fairly distinct for distances of approximately 2 km down-stream of its confluence with the upper Humber and Ouse. These wat ers eventually broke into ribbon-like or streak-like structures within the higher SPM-laden and slightly warmer waters of the Humber. They were discer nible for more than 5 km down-estuary of the confluence of the Humber, Ouse , and Trent. Boat measurements showed that the turbidity maximum occurred o ver a fairly restricted region of the upper Humber, between about 20 to 50 km from the tidal limit at high water. The turbidity maximum's sediment loa d was largely suspended in the water column during stronger currents. SPM r apidly settled close to the bed during high water and low water slack perio ds. At these times, SPM concentrations in a thin, near-bed layer were >60 g l(-1) in the turbidity maximum region of the Ouse and >30 g l(-1) in the u pper Humber (where channel volumes were much greater). SPM within the turbi dity maximum comprised very fine-grained material and its low organic conte nt demonstrated that the SPM was essentially mineral, clastic sediment deri ved originally from erosion and decay of crustal rocks.