RELATIONSHIPS BETWEEN SALT-MARSH LOSS AND DREDGED CANALS IN 3 LOUISIANA ESTUARIES

Coastal land loss rates were quantified for 27 salt marshes in three e stuaries of the Louisiana Mississippi Deltaic plain: Barataria. Terreb onne and St. Bernard. The sites ranged from 23 ha to 908 ha and the to tal area of all sites was 6,367 ha. Two methods were used to calculate open water and canal density in each of five years: (1) a Geographic Information System for 1956 and 1978, and, (2) a point grid method for 1974, 1988, and 1990. A General Linear Model explained 79 % of the va riance (R-2 = 0.79; P greater than or equal to 0.95) among basins for all years and provided an estimate of the impacts of canals in each ba sin. The land loss rates, virtually all occurring as wetland to open w ater conversions, were different in each basin. The 'background' land loss rates from 1956 to 1990 (exclusive of the direct or indirect effe cts of canals; %/yr; mu +/- 1 Std. Dev.) for each basin were estimated to be: Barataria: 0.71 +/- 0.12, Terrebonne 0.47 +/- 0.09, and St. Be rnard 0.08 +/- 0.14. Canals were equally and directly correlated with landless in each basin. There was 2.85 ha of open water formed with ea ch ha of canal dredged (inclusive of the canal area) and an additional 1 ha wetland converted to spoil bank vegetation. Additional losses ma y occur if loss rates continue for periods longer than the mapping int ervals. There are documented causal mechanisms involving wetland hydro logic changes that can explain these wetland losses.