Reconsidering the physics of the Chesapeake Bay estuarine turbidity maximum

A series of cruises was carried out in the estuarine turbidity maximum (ETM ) region of Chesapeake Bay in 1996 to examine physical and biological varia bility and dynamics. A large flood event in late January shifted the salini ty structure of the upper Bay towards that of a salt wedge, but most of the massive sediment load delivered by the Susquehanna River appeared to bypas s the ETM zone. In contrast, suspended sediments delivered during a flood e vent in late October were trapped very efficiently in the ETM. The differen ce in sediment trapping appeared to be due to increases in particle settlin g speed from January to October, suggesting that the fate of sediments deli vered during large events may depend on the season in which they occur. The ETM roughly tracked the limit of salt (defined as the intersection of the 1 psu isohaline with the bottom) throughout the year, but it was often sepa rated significantly from the limit of salt with the direction of separation unrelated to the phase of the tide. This was due to a lag of ETM sediment resuspension and transport behind rapid meteorologically induced or river f low induced motion of the salt limit, Examination of detailed time series o f salt, suspended sediment, and velocity collected near the limit of salt, combined with other indications, led to the conclusion that the convergence of the estuarine circulation at the limit of salt is not the primary mecha nism of particle trapping in the Chesapeake Bay ETM. This convergence and i ts associated salinity structure contribute to strong tidal asymmetries in sediment resuspension and transport that collect and maintain a resuspendab le pool of rapidly settling particles near the salt limit. Without tidal re suspension and transport, the ETM would either not exist or be greatly weak ened. In spite of this repeated resuspension, sedimentation is the ultimate fate of most terrigenous material delivered to the Chesapeake Bay ETM. Sed imentation rates in the ETM channel are at least an order of magnitude grea ter than on the adjacent shoals, probably due to focusing mechanisms that a re poorly understood.