The location of vegetative types in the Louisiana coastal nnarsh was delineated 

 during previous investigations by O'Neil (1949), Chabreck et al. (1968), and Chabreck and 

 Linscombe (1978). Each investigation represented a different time period and provided a 

 base from which temporal changes in vegetative types could be evaluated. 



Changes in the location of the saline and brackish vegetative types over a period of 

 approximately 25 years were determined by comparing the vegetative type map by O'Neil 

 (1949) with that by Chabreck et al. (1968). The saline type in the chenier plain in 

 southwestern Louisiana changed very little over the period and occupied a narrow zone 

 about 0.8 km wide adjacent to the Gulf of Mexico. Comparisons of the saline types in 

 the deltaic plain showed a different situation, however. Measurements from the earlier 

 study may revealed that the saline vegetative type extended inland for an average of 9.3 

 km from the gulf shoreline, but the 1968 map placed this type 12.7 km inland, an 

 encroachment averaging 3.4 km over the 25-year period. 



The brackish vegetative type was also compared on the two maps. Measurements 

 revealed that the brackish marsh extended inland an average of 14.5 km during the 1941- 

 45 period (O'Neil 1949) and 15.6 km in 1968, a retreat of only I.I km. Considerable 

 differences were noted, however, between the chenier plain and deltaic plain marshes. 

 The O'Neil map shows the deltaic plain brackish type extending inland for an average of 

 20.0 km; but, in 1968, the northern boundary of this type was 26.1 km inland. In contrast, 

 the brackish type of the chenier plain extended inland for a mean distance of 9.0 km 

 during the O'Neil study, but by 1968 the northern boundary of this type had advanced 

 seaward to a line only 5.2 km inland. 



Since the saline vegetative type maintained essentially the same position over the 

 years in the chenier plain, the seaward advancement of the northern boundary of the 

 brackish type represents a reduction in the width of this type. In fact, O'Neil (1949) 

 shows the chenier plain brackish type as a strip 8.2 km wide, while Chabreck et al. (1968) 

 shows this same type 4.2 km wide, a reduction of about 47 percent. The brackish type in 

 the deltaic plain, however, actually widened during the 25-year period. During the 

 earlier period, this type was 10.6 km wide, but by 1968, the average width had increased 

 to 13.4 km. 



The widening of the saline and brackish vegetative types in the deltaic plain 

 resulted from saltwater intrusion from the Gulf of Mexico into the intermediate and 

 fresh vegetative types. Increased canal dredging and stream channelization, coupled 

 with subsidence and erosion, were major factors in the change. The reduction in the 

 width of the brackish type on the chenier plain reflected a reduction in water salinities in 

 that area. Factors operating to reduce water salinities during the 25-year interval 

 included the discharge of large amounts of fresh water by the Atchafalaya River into the 

 area plus construction of levees and water control structures to prevent saltwater 

 intrusion. 



Changes in the size of vegetative types in the Louisiana coastal marshes were 

 determined for a 10-year period by comparing the size of types mapped by Chabreck et 

 al. (1968) with those mapped by Chabreck and Linscombe (1978). Chabreck and 

 Linscombe (in press) computed the size of vegetative types and areas where types had 

 changed to either saltier or fresher conditions. They found that vegetative types had 

 changed on 3,730 km'^ or 21.9% of the State's coastal marshland over the 10-year 

 period. This represented a change to saltier vegetative types on 13.7% of the area and to 

 fresher types on 8.2% of the area with a net change to saltier conditions on 5.6% of the 

 entire coastal marshes or 950 km^. 



97 



