WEIRS AND THEIR EFFECTS IN COASTAL LOUISIANA WETLANDS 



(EXCLUSIVE OF FISHERIES) 



R.E. Turner, J.W. Day, Jr., and J.G. Gosselink 



Coastal Ecology Institute 



Center for Wetland Resources 



Louisiana State University 



Baton Rouge, LA 70803 



ABSTRACT 



Weirs have been used for water control in coastal wetlands since the mid-1950's. They were 

 originally proposed for waterfowl management and are now used for salinity and erosion (wetland 

 loss) control. But there are very few field studies of how well weirs work to retard conversion of 

 wetland to open water, and no published studies (peer-reviewed articles in scientific journals) on 

 their impacts except for how they influence waterfowl and fish and invertebrates. Here we 

 summarize the unpublished data results from conferences, symposia, and eight unpublished student 

 theses. 



Water salinity in weired and adjacent unweired marshes is, in general, the same with regards to 

 monthly mean, maximum, or minimum salinities. Weirs cause higher mean water levels, leading to 

 significant soil chemistry changes which may be detrimental to vascular plants and lead to higher 

 rates of marsh breakup. Aquatic vegetation in ponds with weirs may be significantly greater than 

 that found in ponds without weirs. Overwintering waterfowl are apparently attracted to this 

 increased aquatic vegetation. However, from spring to fall, waterfowl and non-game bird densities 

 are the same, or lower, behind weirs compared to control marshes, as are mammal density and 

 hunter usage. 



INTRODUCTION 



Water level management is a common wetland management approach. Fixed crest weirs are 

 one specific water level management tool introduced in the 1950's to benefit waterfowl (Ensminger 

 1963; Cowan et al. 1988). Today, weirs and other water level control devices are commonly built 

 with the hope of reducing coastal wetland loss rates, increasing or changing plant density and 

 composition, sustaining or building up animal populations, and maintaining marsh sed. nentation 

 rates. Since most of the well-documented, high coastal wetland loss rates are related t< artificial 

 hydrologic changes (e.g., Swenson and Turner 1987; Turner 1987; Turner and Cahoon 1987) it is 

 not unexpected that suggestions for corrective measures focus on water level management. The 

 rather severe complications of dramatic landscape changes (Turner 1988) and limited management 

 tools for natural resource managers (at a Federal, State, and landowner level) often result in a 

 resurrection of past practices to adapt to new purposes. At the same time we have very few 

 documented studies of the long-term impacts of any water level management practice in a diversity 

 of coastal environments. Fixed-crest weirs are an exception in that they have been studied in a 

 credible manner, but the results of those studies are generally unpublished. Further, the studies, 

 when cited in permit applications, are often used to support activities inconsistent with the obser- 

 vations discussed in the original report. It is the purpose here to review the original reports on 

 fixed crest weirs and to summarize their apparent impact on water quality, soils, plants, and animals. 



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