very limited use of fixed crest weirs has occurred in fresh marshes. For fresh marshes, where 

 marsh management is an objective of the landowner and a structure is needed, the Soil 

 Conservation Service would normally recommend a variable crest structure for better water 

 management capability. In this study, the landowner specifically requested a fixed crest weir. 



Since research data were lacking on the impacts of fixed crest weirs in fresh marshes, this study 

 was undertaken to gather some baseline data on vegetation and water salinity. The primary 

 objective of the landowner was to improve the area for waterfowl and furbearers. 



MATERIALS AND METHODS 



The study area was a 202-ha tract in at an ecotone of fresh marsh-shrub swamp. The area is 

 east of Avery Island in Iberia Parish, 8 km north of Vermilion Bay. 



The fixed crest weir was installed in September 1981 on what is locally called the "logging canal." 

 The logging canal outlets into Bayou Petite Anse, the major outlet for marsh areas east and north 

 of Avery Island. Creosote sheet pilings and tongue and groove timbers were used for construction 

 of the weir. From September 1981 to July 1983, the weir crest was set at 0.46 m above mean sea 

 level (MSL). From July 1983 to June 1986, the weir crest was set at 0.55 m mean above sea level. 

 Normal marsh elevation averaged 0.61 m above MSL. 



A modified line transect-circular plot sample technique was used to sample vegetation. Foliar 

 cover was of primary interest. The transect line was oriented north to south to traverse the area 

 influenced by the weir. Along the transect line, 0.08-ha sample plots were established at 60.96-m 

 intervals; 12 permanent plots were established. 



Salinity of the water was checked monthly. Samples were taken 91.4 m above and 91.4 m below 

 the weir site. Salinity in parts per thousand (ppt) was determined by using a Yellow Springs 

 Instrument Model 33-S-C-T salinity meter. 



Sampling was conducted from June 1982 through June 1986. Vegetation was sampled once 

 each year and salinity once each month. 



RESULTS AND DISCUSSION 



Vegetation 



In 1982, the vegetative community contained a mix of species representing woody and herbaceous 

 plants. A total of 29 species were recorded in 1982 (Table 1). Dominant plants included Baccharis 

 halimifolia, Sagittaria lancifolia, Sabal minor, and Cladium jamaicense. 



Over time, woody plant cover decreased and herbaceous plant cover increased (Tables 1-5). 

 The coverage of Baccharis halimifolia changed more than any other plant in the 5-year sampling 

 period. Reductions occurred at seven of the sample stations. Another woody species, Myrica 

 cerifera, showed signs of stress as evidenced by branches defoliating and discoloration of remaining 

 leaves. With the reduction in woody plant coverage, there was a corresponding increase in 

 herbaceous plants including Bacopa caroliniana, Scirpus robustus, Eleocharis sp., and Sagittaria 

 lancifolia. Some possible reasons for this were more stable water levels, more sunlight reaching 

 the marsh floor, and less competition. 



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