In the upper reaches of the Chesapeake Bay there occur two distinctive distributions of suspended sediment and associated patterns of sediment transport. During the spring freshet the Susquehanna River overpowers the characteristic net nontidal estuarine circulation in the upper 20 to 30 kilometers of the estuary, and the net flow and sediment transport are seaward at all depths. The marked decrease seaward of the concentration of suspended sediment in the upper bay reveals the close link, during the freshet, between the suspended sediment population and the principal ultimate source of sediment—the Susquehanna River.
With subsiding river flow the net nontidal estuarine circulation is reestablished in the upper bay, and a turbidity maximum is formed. The high concentrations of suspended sediment, greater than those either farther upstream in the source river or farther seaward in the estuary, are produced and maintained primarily by the periodic resuspension of bottom sediment by tidal scour and by the sediment trap created in the upper reaches of the estuarine circulation regime.