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Abstract
Apalachicola Bay: Dynamic Sedimentation in a Gulf Coast Estuary
Wayne C. Isphording (1)
ABSTRACT
Nineteen major estuaries are present in the coastal zone of the northern Gulf of Mexico between Brownsville, Texas and the southern tip of Florida. While the sediments deposited in these basins are derived, in part, from reworking of offshore Pleistocene and recent deposits, each can be shown to be dominated by river-borne detritus. Past studies have shown that the great majority of material carried by rivers is in the form of suspended load. Further, most of the sediment actually moved into estuaries by rivers, falls into the silt and clay size range. Analysis of river sediment being carried into estuaries in the Gulf Coast (and elsewhere in coastal United States) would lead to the conclusion that sand-sized material is relatively unimportant, constituting not more than five percent of the total transported material.
In view of this, Apalachicola Bay, Florida must be considered a unique depositional basin. This, because not only does it evidence depositional rates that are several times greater than those for other Gulf Coast estuaries, but also because the sediments are rich in sand and largely devoid of a silt component. This discovery is even more striking when older bay sediments are examined. Samples corresponding to depths approximating the 1825 and 1900 depositional surfaces revealed that silt was a common constituent in the past. Further, silt was apparently a common component of the sediments as recently as 1959. The abrupt change in the sediment regimen over the past 25 years can therefore only be traced to a man-induced cause.
During the late 1950's and the 1960's, a number of dams were completed on rivers emptying into Apalachicola Bay. Though the nearest dam is over 100 miles upstream, these offer the best explanation for the sedimentation changes that have taken place. It is well known that silt and sand become trapped in the reservoirs behind dams while the finer grained, suspended particles are washed over the spillways and continue downstream. Further, as silt and sand are removed from the stream by this process, the stream's erosive power is increased and downstream widening and channel deepening below dams is often observed. Evidence strongly points to such activity in the Apalachicola River. While the total quantity of suspended sediment delivered to the bay has not changed since the 1950's, a greater contribution of the river's suspended and traction load is now being supplied from increased erosion downstream from the last dam. As a consequence of this, Apalachicola Bay is now undergoing in-filling at a greatly accelerated rate.
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