Effects of weir management on marsh loss,Marsh Island,Louisiana, USA

Weirs are low-level dams traditionally used in Louisiana's coastal marshes to improve habitat for ducks and furbearers. Currently, some workers hope that weirs may reduce marsh loss, whereas others fear that weirs may accelerate marsh loss. Parts of Marsh Island, Louisiana, have been weir-managed since 1958 to improve duck and furbearer habitat. Using aerial photographs, marsh loss that occurred between 1957 and 1983 in a 2922-ha weir-managed area was compared to that in a 2365-ha unmanaged area. Marsh loss was 0.38%/yr in the weir-managed area, and 0.35%/yr in the unmanaged area. Because marsh loss in the two areas differed less than 0.19%/yr, it was concluded that weirs did not affect marsh loss. The increase in open water between 1957 and 1983 did not result from the expansion of lakes or bayous. Rather, solid marsh converted to broken marsh, and the amount of vegetation within previously existing broken marsh decreased. Solid marsh farthest from large lakes and bayous, and adjacent to existing broken marsh, seemed more likely to break up. Marsh Island has few canals; therefore, marsh loss resulted primarily from natural processes. Weirs may have different effects under different hydrological conditions; additional studies are needed before generalizations regarding weirs and marsh loss can be made.