VEGETATION AND SALINITY CHANGES FOLLOWING THE 



INSTALLATION OF A FIXED CREST WEIR AT 



AVERY ISLAND, LOUISIANA (1982-86) 



Billy R. Craft and Dolan Kleinpeter 



Soil Conservation Service 



Alexandria, LA 71302 



ABSTRACT 



Fixed crest weirs have been extensively used in Louisiana's coastal area. This study was initiated 

 primarily to document the impacts of a fixed crest weir on vegetative communities and water 

 salinities. The duration of this study extended from June 1982 through June 1986. The study area 

 is an ecotone of fresh marsh-shrub swamp comprising about 202 ha and is subject to daily tidal 

 influence. 



The fixed crest weir was installed in September 1981. It was constructed of creosote sheet piling 

 and tongue and groove timbers. Length of the weir crest was 6.1 m. 



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

 Foliar cover was of primary interest. The transect line was orientated in a north-south direction 

 to transverse the area influenced by the weir. Along the transect line, 0.08-ha sample plots were 

 established at 60.96-m intervals. 



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 was determined by using a Yellow Springs Instrument 

 Model 33-S-C-T salinity meter. 



Noticeable changes occurred in the plant community. In 1982, there were 29 species of plants 

 recorded; in 1986, 35 species. Over the study period, woody plant cover decreased and herbaceous 

 plant cover increased. Generally, plants beneficial to wildlife increased in abundance. 



The coverage of Baccharis halimifolia changed more than any other plant. There was a 

 reduction of eastern baccharis at seven of the sample stations. This allowed herbaceous plants such 

 as Bacopa caroliniana, Eleocharis sp., and Scirpus robustus to increase, possibly because of more 

 sunlight, less competition, and more stable water levels. 



Salinity differences were evident in a comparison of upstream and downstream results. Monthly 

 salinity was determined for 56 months. For 41 months (73%), salinity was less upstream of the 

 weir; for 12 months, salinity was the same upstream and downstream of the weir, and for 3 months, 

 salinity was greater upstream of the weir. 



Based on the objective of the landowner, which was to improve habitat for waterfowl and 

 furbearers, the fixed crest weir is serving to accomplish its purpose and function. 



INTRODUCTION 



Fixed crest weirs have been extensively used in Louisiana's coastal brackish and intermediate 

 marshes (Chabreck and Hoffpauir 1962; Chabreck 1967; Larrick and Chabreck 1976). However, 



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