It is not clear exactly when weirs were first used in coastal Louisiana. Arthur (1928) made no 

 mention of weirs in his book on Louisiana fur animals. However, O'Neil (1949) presented a 

 irr.rsh management plan that called for low dams in access ditches to prevent drainage of the 

 marsh but to allow high tide to flood it as usual. The purpose was to prevent vegetative changes 

 in the marsh because of drainage by the ditches. Weir construction began in the 1940's or early 

 1950's, and we believe the period of most construction was 1955-65. During that time weirs were 

 constructed throughout the coastal marshes, but many were concentrated in the brackish and 

 intermediate marshes of the Deltaic Plain. 



The purpose of this paper is to review the effects of weirs on plants and wildlife in coastal 

 Louisiana. We have limited our discussion to studies of fixed-crest weirs. Weirs used in 

 conjunction with levees or with an adjustable height or a vertical slot are not included because 

 comprehensive studies have not been conducted on the effects of these structures on plants and 

 wildlife. We have also limited this review to studies in which a similar unmanaged marsh area 

 (control) was compared to the marsh influenced by weirs. 



LITERATURE REVIEW 



Effects on Aquatic Vegetation 



Because abundant aquatic vegetation provides important food for ducks (Bellrose 1976), one 

 of the purposes of weirs is to increase the amount of aquatic vegetation. Chabreck and Hoffpauir 

 (1962) recorded the distribution of aquatic vegetation on Marsh Island in Iberia Parish and in 

 Terrebonne and Lafourche Parishes. On Marsh Island, they found that the species composition 

 of aquatic vegetation varied with water salinity. Drastic changes in salinity were associated with 

 discharge from the Atchafalaya River. Managed ponds and control ponds were nearly identical in 

 aquatic vegetation, salinity, and turbidity at the time of construction. On five of the subsequent 

 nine sampling dates, salinities were relatively higher and widgeongrass (Ruppia maritima) was the 

 dominant vegetation in managed ponds. On these sampling dates aquatic vegetation occurred at 

 11% of the stations in managed ponds and 6% of the stations in free-draining (control) ponds, and 

 widgeongrass was the dominant species in the free -draining ponds on four of these occasions. On 

 the other four sampling dates aquatic vegetation occurred at 3% of the stations in managed ponds 

 and 4% of the stations in free-draining ponds. In managed ponds the dominant species was wild 

 celery (Vallisneria spiralis) and in unmanaged ponds it was either wild celery, pondweed 

 (Potamogeton foliosus), or dwarf spikerush {Eleocharis parvula). 



The study areas in Terrebonne and Lafourche Parishes were sampled only once and contained 

 weirs of various ages. Widgeongrass occurred at 41% of stations in managed ponds and 11% of 

 the stations in unmanaged ponds. There was no relationship between the age of the weirs and the 

 occurrence of widgeongrass. 



In the Terrebonne-Lafourche area, turbidities in managed and unmanaged ponds were less than 

 25 ppm. On Marsh Island, considerable variations in turbidity were noted throughout the study, 

 but generally turbidity was less in managed ponds than unmanaged ponds. Little difference in 

 salinity was found between managed and unmanaged ponds in either area. 



At the Biloxi Marsh Tract in St. Bernard Parish, the Louisiana Wildlife and Fisheries Commission 

 (1964-65) found 10.7 ml of aquatic vegetation per sample in managed ponds and 2.4 ml in 

 unmanaged ponds in 1964; and 22.4 ml in managed ponds and 5.1 ml in unmanaged ponds in 1965. 

 No difference was noted in turbidity, and the difference in water salinities (4.3 ppt in managed 



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