responsible for a large part of the 

 marsh currently being lost in the basin. 



Regional shifts in the source and 

 seasonality of sedimentation in marshes 

 influence marsh accretion rates. During 

 the past 5,000 years, direct introduc- 

 tion of riverine sediments during spring 

 floods supplied marshes in the active 

 delta lobe with enough sediment to 

 counter the effects of subsidence, and 

 provided a surplus that enabled marsh 

 accretion to occur. During the same 

 time period, newly abandoned delta lobes 

 suffered a gradual decline in direct 

 sedimentation for several hundred years. 

 Much of the MDPR wetland area lies in 

 abandoned delta lobes, and the construc- 

 tion of levees along the Mississippi 

 River is exacerbating the natural reduc- 

 tion in sediment supply. 



With the exception of the Atcha- 

 falaya Bay area, most of the MDPR 

 receives a relatively low rate of sedi- 

 ment input , much of which arrives during 

 storms and hurricanes. Storm events do 

 not introduce new sediment but rework 

 and redistribute bay-bottom sediments. 

 Baumann (1980) found that 41% of all 

 sedimentation from 1975 to 1979 in 

 Barataria saline marsh areas ocurred 

 during the winter months; 33% was the 

 result of two tropical storms that 

 passed through the area, and 21% oc- 

 curred during the remaining months. 



Storm activity is the key to high 

 sedimentation rates during winter 

 months. Southerly winds increase as a 

 cold front approaches from the north- 

 west, raising water level in the marsh 

 and increasing the suspended sediment 

 load in the water. Some of this sus- 

 pended material is deposited on the 

 marsh surface. As the front passes, 

 winds swing to a northerly direction, 

 causing a rapid drop in water level 

 (Baumann 1980) . 



Cultural Processes and Wetland Loss 



A diverse array of human activities 

 have contributed to wetland loss in the 

 MDPR. These include canals for oil 

 exploration, pipelines, navigation 

 channels, recreational developments, 



drainage channels, and flood control. 

 Other cultural factors contributing to 

 wetland loss are construction of bulk- 

 heads and seawalls, highways, and im- 

 poundments. Evidence is increasing that 

 canals are an important factor leading 

 to wetland loss in the MDPR. 



Canals are now a very common fea- 

 ture of the MDPR (see the canal section 

 under HABITAT DESCRIPTIONS). Canal area 

 (Figure 22), excluding the area of spoil 

 deposition, is about 1.4% of the Lou- 

 isiana coastal zone, or about 20,200 ha 

 (50,000 acres) (Craig et al. 1979). 

 Canals directly affect wetlands by 

 excavation and spoil deposition. They 

 indirectly impact wetlands by altering 

 hydrology, sedimentation, and produc- 

 tivity. Canals have accelerated 

 salinity intrusion causing marsh vege- 

 tation loss (Van Sickle et al. 1976). 

 Canal spoil banks limit water exchange 

 over wetlands and decrease deposition of 

 suspended sediments. This exacerbates 

 the existing sediment deficit in inland 

 marshes. Sedimentation is also the 

 single most important source of nutri- 

 ents added to the marshes (Delaune and 

 Patrick 1980a) . Lack of sediment 

 therefore leads to lower productivity. 

 Craig et al. (1979) estimated that 

 almost 70% of the total annual loss of 

 102 km 2 (39.4 mi 2 ) in the MDPR may be 

 the direct or indirect result of canals 

 and canal building. They also conclude 

 that 10% of the direct loss of wetlands 

 is attributable to canals in the Bara- 

 taria basin, the area for which the best 

 data exist. Johnson and Gosselink 

 (1982) also estimated that canals were 

 responsible for 10% of the direct loss 

 of MDPR wetlands during the period from 

 1955-1978, and 20% if spoil bank area is 

 included (see below) . 



Once established, canals tend to 

 widen over time, especially as a result 

 of wave action and altered hydrologic 

 patterns (Craig et al. 1979; Johnson and 

 Gosselink 1982). The annual increase in 

 canal width varies from 2% to 14%, for a 

 doubling time ranging from 7 to 50 

 years. As the cross sectional area and 

 density of canals increase, the cross 

 sectional area and density of natural 

 channels decrease, indicating altered 



153 



