THE EFFECTS OF WEIRS ON PLANTS 

 AND WILDLIFE IN THE COASTAL MARSHES OF LOUISIANA 1 



Robert H. Chabreck and J. Andrew Nyman 



School of Forestry, Wildlife, and Fisheries 



Louisiana Agricultural Experiment Station 



Louisiana State University Agricultural Center 



Baton Rouge, LA 70803 



ABSTRACT 



An important technique for management of tidal marsh, particularly in areas that will not support 

 levees to form impoundments, is construction of weirs in drainage systems of the marsh. A weir 

 resembles a low dam constructed of steel or wooden sheet pilings. The top or crest of the weir 

 is normally placed 15 cm below the elevation of surrounding marsh, and water is allowed to flow 

 back and forth across the structure. A weir reduces the rate of tidal flow and establishes a basin 

 of water behind the structure that cannot recede below the crest; consequently, complete drainage 

 of marshes and most ponds on low tide is prevented. With reduced tidal flow, streams carry less 

 suspended materials, and water turbidity decreases. The basin of water held by the weir functions 

 as a mixing bowl and stabilizes water salinity. Hundreds of weirs have been constructed along the 

 Louisiana coast for marsh management. Studies comparing ponds and lakes behind weirs with 

 those drained by free-flowing streams (control areas) disclosed that production of aquatic plants 

 was 400% greater and more ducks were present in ponds behind weirs. Stabilized water levels 

 improve access for trappers, hunters, and other visitors to a marsh. 



INTRODUCTION 



A weir is a sheet-piling bulkhead placed across a drainage outlet; it prevents complete drainage 

 of the marsh during lowest tides but allows the marsh to be flooded normally at high tide. The 

 crest of the weir is usually set 15 cm below the elevation of the marsh. The basin created by a 

 weir is often referred to as a semi-impoundment or a partial impoundment (Herke 1971; Day et 

 al. 1986). 



Weirs are used in coastal Louisiana for several reasons. Their primary purpose is to increase 

 the amount of habitat available to wintering waterfowl. Extreme low tides are very common in 

 the coastal marshes during winter months and may last 3 to 4 days. Free-flowing ponds are drained 

 by north winds, but ponds upstream of weirs remain flooded and available to waterfowl (Chabreck 

 1968). Another major role of weirs is to maintain usable waterways through the marsh during the 

 winter to provide trappers unrestricted access. Weirs increase production of aquatic plants, which 

 are often high quality waterfowl foods. A fourth purpose of weirs is to favor the growth of 

 muskrat (Ondatra zibethicus) and nutria (Myocastor coypus) foodplants. Diets of these economically 

 important furbearers consist primarily of emergent vegetation of the genera Scirpus and Eleocharis. 

 Weirs prevent excessive drainage and saltwater intrusion because of artificial channelization of the 

 marsh. Saltwater intrusion often kills vegetation and causes marsh loss. 



Approved for publication by the Director of the Louisiana Agricultural Experiment Station as MS. 

 89-22-3116. 



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