Since 1991, Mississippi River water has been diverted at Caernarvon,Louisiana, into Breton Sound estuary. Breton Sound estuary encompasses 1100 km2 of fresh and brackish, rapidly subsiding wetlands. Nitrite + nitrate, total Kjeldahl nitrogen, ammonium, total phosphorus, total suspended sediments, and salinity concentrations were monitored at seven locations in Breton Sound from 1988 to 1994. Statistical analysis of the data indicated decreased total Kjeldahl nitrogen with associated decrease in total nitrogen, and decreased salinity concentrations in the estuary due to the diversion. Spring and summerwater quality transects indicated rapid reduction of nitrite + nitrate and total suspended sediment concentration as diverted MississippiRiver water entered the estuary, suggesting near complete assimilation of these constituents by the ecosystem. Loading rates of nitrite +nitrate (5.6-13.4 g m-2 yr-1), total nitrogen (8.9-23.4 g m-2 yr-1), and total phosphorus (0.9-2.0 g m-2 yr-1) were calculated along with removal efficiencies for these constituents (nitrite + nitrate 88-97%; total nitrogen 32-57%; total phosphorus 0-46%). The low impact of thediversion on water quality in the Breton Sound estuary, along with assimilation of TSS over a very short distance, suggests that more water may be introduced into the estuary without detrimental affects. This would be necessary if freshwater diversions are to be used to distribute nutrients and sediments into the lower reaches of the estuary,in an effort to compensate for relative sea-level rise, and reverse the current trend of rapid loss of wetlands in coastal Louisiana. [ABSTRACT FROM AUTHOR]