(Sasser 1977; Baumann 1980), the land slope 

 is also exceedingly snail. The slope of 

 the water is slightly steeper in the 

 Atchafalaya basin because of the enormous 

 river inflow. Generally, across the coast 

 it is so slight that "downhill" changes 

 daily, depending on the astronomical tide 

 stage, wind direction and strength, rain- 

 fall, local runoff, and river flow. 



On a smaller scale of meters rather 

 than kilometers, a slope also exists on the 

 marsh surface from the edge of tidal 

 streams inland. Water overflowing stream 

 banks on flood tides slows and drops much 

 of its sediment load near the stream edge 

 as it moves inland, creating a slight 

 crest or levee next to the stream. 

 Because of this, water tends to drain 

 away fran streams into small marsh chan- 

 nels that eventually carry the water back 

 through the natural levee. The natural 

 creekbank levee, which is usually measured 

 in centimeters, and the slight marsh sur- 

 face slope are enough to create a gradient 

 of inundation, water chemistry and biotic 

 activity. These hydraul ical ly mediated 

 gradients dre responsible for much of the 

 observed biotic diversity in the delta 

 marshes. 



F1 ooding 



Information on the frequency and 

 duration of marsh flooding is rather 

 scarce. Sasser (1977) and Baumann (1980) 

 measured marsh elevations relative to 

 local mean water levels and calculated 



inundation statistics for a number of 

 different species and associations from 

 nearby tide gauge records. Byrne et al. 

 (1976) plotted frequency and duration of 

 flooding at locations in the Barataria 

 basin corresponding to salt, brackish and 

 fresh marshes. They did not measure the 

 elevation of any marshes relative to these 

 data. However, by interpolating Sasser's 

 elevations on the graphs by Byrne et al . 

 it is possible to come up with several 

 estimates of marsh inundation (Table 7). 



Considering the variability in these 

 estimates, it appears that the total 

 duration of flooding during the year is 

 about constant across the whole marsh from 

 coast to upland. But the regular, daily 

 tidal flushing of the salt marsh is 

 replaced by a more infrequent flooding 

 inland where wind tides and upstream 

 runoff play a much larger role. The delta 

 marshes appear to be flooded about 50 per- 

 cent of the time. The average duration of 

 a flooding increases from 12 to 16 hours 

 at the coast to almost 5 days in fresh 

 marshes. Notice that the streamside 

 marsh, some 10 - 15 an above the inland 

 marsh, is inundated almost as often but 

 for much shorter time periods, so that it 

 is flooded only about 12 percent of the 

 year. 



Baumann (1980) showed that inundation 

 characteristics are not constant 



throughout the year (Figure 30). Flooding 

 frequency does not vary much, but because 

 the water level varies seasonally, the 



Table 7. The annual duration and frequency of inundation of marshes in 

 the Barataria basin, Louisiana. Figures in parentheses indicate the 

 percentage of the year inundated. 



33 



