The extent and evolving pattern of new subaerial marsh in the Atchafalaya delta 

 lobe is illustrated in Figure 6. Unusual hydrologic conditions during the first 3 years of 

 subaerial exposure played an important role in the rapid development of this dynamic 

 phose of Atchafalaya Delta growth. Rouse et al. (1978) showed that by early 1976, 19.0 

 km (7.3 mi ) of new Iqnd had forrned above mean sea level, corresponding to an average 

 growth rate of 4.75 km /yr (1.8 mi^/yr) (Figure 7). Through aerial-photo mapping of the 

 eastern half of the delta, van Heerden (1980) confirmed the dramatic growth rate in 

 1973, 1974, and 1975 and the major flood in 1979. During average floods the growth rate 

 is somewhat reduced, however. 



Through analysis of LANDSAT imagery a growth curve has been developed for Wax 

 Lake delta lobe (Figure 8). Unpublished data (Susan Chinburg, Coastal Studies institute, 

 Louisiana State University, Baton Rouge, 1981, personal communication) suggest that the 

 Atchafalaya Delta exhibits the same growth trends, although on a larger scale. Subaerial 

 expression of new marsh land increased steadily from 1973 to 1976, but decreased during 

 1977 and 1978. This reduction in surface area reflects the average-sized floods during 

 these years, but more importantly reveals the dynamic effects of wind-wave-induced 

 erosion during the passage of winter cold fronts (van Heerden and Roberts 1980a). The 

 cumulative effects of the passage of cold fronts spaced at approximately I -week 

 intervals are erosion and denudaton of new marsh surface. During minor floods this loss 

 may not be completely replenished. During major floods, however, the marsh surface 

 aggrades significantly, offsetting any land loss resulting from cold-front-related erosion. 



DELTA LOBE RESPONSE CHARACTERISTICS 



Systematic monitoring of land accretion, changes in channel cross sections, and 

 sediment characteristics have shown that delta growth responds directly to flood volume 

 and duration. Reductions in channel cross section are most dramatic during major floods 

 (van Heerden and Roberts 1980b). Distributary channels experience mid-channel shoaling 

 and bar formation at their seaward ends (Figure 9). This bifurcation mechanism results 

 in a complex network of sand lobes, separated by branching distributaries, characteristic 

 of deltas whose river mouths are frictionally dominated and are generally building into 

 unstratified, low-energy, shallow-water environments (Welder 1959; Wright and Coleman 

 1974). 



As the fluvial effluent passes from the confined distributary channel to the shallow, 

 unconfined bay, it rapidly experiences a reduction in velocity. Associated with the 

 frictional deceleration of the flow is a reduction in turbulence and the coarsest part of 

 the suspended load is deposited, initiating a mid-channel bar (Figure lOa). Once 

 initiated, shoaling hxiyward of the mouth causes an increase in the friction-induced 

 deceleration and effluent spreading, which in turn increases the shoaling rate (Bates 

 1953; Wright 1977). The overall effect of the differential sedimentation is a branching of 

 the channel into two distributaries (Figure I Ob). Velocities also decrease away from the 

 center line of the divergent current field. Deposition occurs at the outer edges of the 

 effluent plume, giving rise to subaqueous levees. The levee ridges flare away from the 

 mouth, reflecting the divergent current field that results from the abrupt transition to 

 unconfined flow (Figure lOc). The same process may then be repeated on the two newly 

 formed channels (Figure lOd). In the above manner, the subaerial components of the 

 emergent delta have evolved into a complex network of sand lobes separated by 

 branching distributaries. 



221 



