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will be re -entrained throughout most of the length of the mixing zone to 

 levels above the salt water- fresh water interface and will be transported 

 downstream to form larger aggregates once again, and these will settle as 

 before . At the seaward end some material may be transported out of the 

 system. A portion or all of this could ultimately return with the net 

 upstream current. 



Sediment moving upstream along the bottom may also be derived from 

 marine sources, as noted. The strength of this upstream current is often 

 enhanced by the inequality between the flood and the ebb flows induced by 

 the usually observed distortion of the tidal wave. Inasmuch as the low 

 water depth is often significantly less than the depth at high water, the 

 speed of the propagating tidal wave is higher at high water than at low 

 water. This typically results in a higher peak flood velocity than peak 

 ebb velocity and a shorter flood period than ebb period. Such a situation 

 tends to enhance the strength of the upstream bottom current, and the 

 sediment is sometimes transported to regions upstream of the limit of 

 seawater intrusion. 



The estuarine sedimentary regime is characterized by several periodic 

 (or quasi-periodic) time scales. These are: 1) the tidal period (diurnal, 

 semi-diurnal, or mixed), 2) lunar (spring-neap) cycle, 3) yearly cycle, and 

 4) periods greater than a year. Of these, the first is the fundamental 

 period which characterizes the basic mode of the sediment transport 

 phenomenon in an estuary. The second is important from the point of view 

 of determining net shoaling rates in many cases of engineering interest, 

 and by the same token the third and the fourth time -scales are involved in 

 considerations of long-term stability and shoaling in estuaries, as for 

 example due to sea level rise. 



Predictive capability for estuarine sedimentation can be illustrated 

 by considering two case studies, Atchafalaya Bay, Louisiana, and Savannah 

 River estuary, Georgia. The Atchafalaya River, a distributary of the 

 Mississippi River, discharges into this bay. In recent years, the delta at 

 the mouth of the river has grown dramatically. A study of the bay and 

 adjacent waters was carried out to predict the rate at which the delta will 

 evolve in the short term (<10 years) and the long term (50 years), and the 

 manner by which that evolution will affect flood stages, navigation channel 



