WATER AND SEDIMENT INPUT 



Thirty-four years of hydrographic data collected on Atchafoloya River flow at 

 Simmesport, Louisiana, show that the average annual flow over the sample period (1938- 

 72) was 5,126 rn^/s (181,000 U^/s) (U.S. Army Corps of Engineers 1974). Within this data 

 collection period, the average annual peak flow that occured in the spring was 

 approximately I 1,300 m-^/s (400,000 U^/s). About 70% of this flow arrived at the coast 

 through the Lower Atchafalaya River Outlet, while the remainder was transported 

 through the man-made Wax Lake Outlet. During the years of subaqueous delta growth 

 (early I950's to 1972), flood levels only occasionally exceeded the I 1,300 m-'/s (400,000 

 ft /s) level (Figure 2); from 1973 to 1980, however, this level was significantly exceeded 

 three times, in 1973, 1975, and 1979. These abnormal floods also transported a 

 proportionately higher-than-average sediment load to Atchafalaya Bay. Flow velocities 

 during flood are such that the coarsest particles available (generally fine-sand size) can 

 be transported as suspended load (Roberts et al. 1980). In response to abnormally high 

 discharge during the 1970's, deposition and subsequent subaerial growth of the 

 Atchafalaya Delta have been impressive, as is illustrated by a 1976 photomosaic (Figure 

 3). The most recent flow measurements (1979-81) made in the lower reaches of 

 Atchafalaya River and in the main arteries of the newly formed delta indicate that 

 approximately 67% of the water and sediment transported from the Lower Atchafalaya 

 River mouth goes down the western branch (dredged navigation channel), while about 

 27% is conducted through the eastern branch (Figure 3). Minor passes near the river 

 mouth account for the remaining 6% of the flow. 



Roberts et al. (1980) present a sediment budget for the Atchafalaya system from 

 1967 to 1975; the annual suspended sediment load nearly doubled during the three 

 high-water years of the early 1970s. It was estimated that much of the suspended-load 

 sand was derived from scouring and resuspension of previously deposited sediments in the 

 Lower Atchafalaya River course. The net change in the dominance of sediment reaching 

 the bay from clay and silt to silt and fine sand over the last 30 years has resulted in the 

 construction of sizable sand-rich sediment lobes that have been rapidly colonized by 

 marsh plants as soon as they build to the low-tide level. 



SPATIAL-TEAAPORAL CHANGES IN MARSH LAND 



Bathymetric changes in Atchafalaya Bay have been impressive. The 1967 

 bathymetric map shows distributary-mouth bar deposits whose limits are roughly 

 represented by the 4-ft (1.2-m) depth contour. At this time these deposits were 

 beginning to prograde into the bay, forming broad, shallow platforms which front the 

 natural channels of Lower Atchafalaya River and Wax Lake outlets (Figure 4). By 1972 

 the distributary-mouth bar platform had extended over most of the bay (Roberts et al. 

 1980). The natural channel of the Lower Atchafalaya River mouth showed a pronounced 

 seaward extension and development of a major bifurcation to the east. 



The 1977 bathymetric map of Atchafalaya Bay (Figure 5) emphasizes the 

 tremendous volume of predominantly coarse-grained material deposited in the decade 

 1967-77. An extensive network of distributary-mouth bar deposits formed in both the 

 complex Wa>eLake and Atchafalaya delta lobes. Roberts et al (1980) estimated that 16 

 km^ (6.55 mi^) of new land had developed above mean sea level by 1977. When estimated 

 from the low tide level, a net land gain of 32.5 km^ (12.6 mi ) over the same period was 

 calculated (Rouse et al. 1978). 



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