natural conditions." Impoundment or 

 filling of wetlands does not eliminate 

 an area of land (i.e., by converting it 

 to water), but it does eliminate the 

 natural ecological function of the wet- 

 land. In this report wetland loss in- 

 cludes the conversion of wetland either 

 to water or to dry land. 



Before man intervened, wetland loss 

 in abandoned delta lobes was compensated 

 for by land building in the active 

 delta. Now there has been a dramatic 

 shift in the KDPR from net land gain to 

 net loss (Gagliano and van Beek 1970; 

 Adams et al. 1976; Craig et al. 1979; 

 and Gagliano et al. 1931). Recent 

 measurements show that the rate of loss 

 is accelerating and the latest estimate 

 is 102 km2 (39.4 mi2) annually for the 

 MDPR (Gagliano et al. 1931). There are 

 both natural and cultural reasons for 

 wetland loss, both of which are sum- 

 marized below (see also THE PHYSICAL 

 SETTING) . 



Natural Wetland Loss 



Three major natural processes con- 

 tribute to wetland loss: (1) Gulf of 

 Mexico beach retreat, (2) lateral ero- 

 sion of streamside marsh shores, and (3) 

 gradual sinking of inland marshes. Wave 

 attack is the primary cause of shoreline 

 retreat. Lack of sufficient sediments 

 to offset apparent sea level rise causes 

 inland marsh loss. Apparent sea level 

 rise in Louisiana is due to both region- 

 al subsidence and eustatic sea level 

 rise (see THE PHYSICAL SETTING). 



Subsidence has occurred steadily 

 during the past 130 years over the en- 

 tire northern gulf coast, with a signif- 

 icant rate of increase during the past 

 25 years. From 1848 until 1959, the 

 measured rate of subsidence from two 

 locations in or adjacent to Barataria 

 basin was 0.27 and 0.83 cm /yr (or 0.11 

 and 0.33 in/yr, respectively). From 

 1959 to 1971, these rates increased to 

 1.29 and 1.12 cm/yr (0.5 and 0.44 in/yr) 

 respectively (Swanson and Thurlow 1973). 

 A third location in the nearshore gulf 

 experienced a subsidence rate of 1.51 

 cm/yr (0.59 in/yr) from 1959 to 1971. 

 In the most recent analysis in the MDPR, 



Baumann (1980) found that subsidence in 

 the lower Barataria basin was 1.30 cm/yr 

 from 1954 to 1979. 



Deposition of river sediment in 

 wetlands causes aggradation that serves 

 to counteract the effects of apparent 

 sea level rise. Studies show, however, 

 that in many areas sedimentation is not 

 great enough to compensate for subsi- 

 dence. Delaune et al. (1978) found that 

 marsh sites near natural streams 

 (streamside marshes) accrete at 1.35 

 cm/yr (0.53 in/yr), while marsh sites 

 away from tidal streams (inland marshes) 

 accrete at only 0.75 cm/yr (0.29 in/yr). 

 They observed that much of the inland 

 marsh in the study sites was accreting 

 at rates less than the rate of apparent 

 sea level rise. When compared with the 

 subsidence rates mentioned above (1.29 

 and 1.12 cm/yr) it is evident that only 

 the streamside marsh is accreting fast 

 enough to offset the effects of subsi- 

 dence. Suspended sediment must pass 

 through the streamside marsh before it 

 reaches the inland marsh. Boto and 

 Patrick (1978) found that streamside 

 marsh grass develops greater stem den- 

 sity than inland marsh, which enhances 

 sediment removal. Newly deposited 

 sediment provides nutrients that allows 

 streamside marsh to maintain higher 

 levels of net primary production than 

 inland marsh (Delaune and Patrick 

 1980a). 



Baumann (1980) observed similar 

 patterns of accretion in his analysis of 

 sedimentation. The accretion rate of 

 streamside marshes (1.52 cm/yr or 0.59 

 in/yr) was significantly higher than 

 inland marsh accretion rates of 0.91 

 cm/yr (0.35 in/yr). For all inland and 

 streamside sites that had "aggradation 

 deficit" (sedimentation rate less than 

 apparent sea level rise) , Baumann found 

 a weighted average sedimentation rate of 

 1.12 cm/yr (0.44 in/yr). Compared with 

 the subsidence rate of 1.30 cm/yr (0.51 

 in/yr) , this leaves a mean aggradation 

 deficit of 0.18 cm/yr (0.07 in/yr). 

 Eighty percent of the marsh in the 

 Barataria basin is experiencing an 

 aggradation deficit. Baumann proposed 

 that this deficit indicates that lack of 

 adequate external sediment supply may be 



152 



