Effects of Tropical Storm Agnes on the Suspended Solids of the Northern Chesapeake Bay
In the upper reaches of the northern Chesapeake Bay there are two distinctive distributions of suspended sediment and associated patterns of sediment transport. During the spring freshet, the Susquehanna River overpowers the characteristic net non-tidal estuarine circulation in the upper 20–30 km of the estuary and the net flow and sediment transport are seaward at all depths. Generally the hulk — probably 70 to 75 percent — of each year’s supply of new fluvial sediment is introduced during the spring freshet when both river flow and concentration of suspended sediment are normally highest. The marked decrease seaward of the concentration of suspended solids in the upper bay reveals the close link, during the freshet, between the suspended sediment population and the principal “ultimate” source of fluvial sediment—the Susquehanna river.
With subsiding river flow, the net non-tidal estuarine circulation is re-established in the upper reaches of the bay and a turbidity maximum is formed near the head of the estuary. The high concentrations of suspended solids, 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.
The passage of tropical storm Agnes in June 1972, resulted in record flooding throughout the drainage basin of the northern Chesapeake Bay. On June 24, the day the Susquehanna crested at its mouth, the instantaneous peak flow exceeded 32,000 m3/sec. The daily average discharge of 27,750 m3/sec. for that day exceeded the previous daily average high by nearly 33 percent. Throughout the bay, salinities were reduced to levels lewer than any previously observed. On June 26, 1972, salinities were less than 0.5% from surface to bottom throughout the upper 60 km of the bay, and the surface salinity was less than 1% in the upper 125 km of the bay. Salinities remained low throughout most of the summer, but had nearly recovered to normal levels by September.
On June 24, the concentration of suspended solids at the mouth of the Susquehanna River exceeded 10,000 mgl-1 and, in a one-week period, the sediment discharge exceeded that of the past several decades. The bulk of this sediment was deposited in the upper 40 km of the bay.
KeywordsSuspended Solid River Flow Total Suspended Solid Tropical Storm Turbidity Maximum
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