This dissertation focuses on impact on water quality of two freshwater diversions, the Bonnet Carre Spillway and the Caernarvon freshwater diversion, as well as the Atchafalaya River Estuarine Complex. As water passed through the Bonnet Carre Spillway, there were reductions in total suspended sediment concentrations of 82-83%, in nitrite+nitrate of 28-42%, in total nitrogen (TN) of 26-30%, and in total phosphorus (TP) of 50-59%. The Si:N ratio generally increased and the N:P ratio decreased from the river to the plume edge in Lake Pontchartrain. At the Caernarvon diversion, there were reductions in concentrations of dissolved inorganic Si, N, and P of up to 38%, 57% and 23%, respectively. The DSi:DIN ratio rose from 0.9 to 2.6 at the Gulf end member station, while the DIN:DIP ratio fell from 107 to 26. There were decreases in total suspended sediment, large changes in salinity correlated to diversion discharge, and decreased water temperatures associated with riverine discharge. Chlorophyll concentrations near the diversion were low due to light limitation, but increased after suspended sediments decreased below 80 mg L-1, then decreased Gulfward. In the Atchafalaya River estuarine complex, salinity fluctuated seasonally, with the lowest salinities occurring during high river discharge. There was a 41-47% decrease in NO3- concentrations and total suspended sediments decreased as river water flowed through the estuarine complex.
In summary, the following conclusions can be made from these studies of the effect on water quality of riverine input into coastal wetlands: (1) riverine input has a pronounced effect on salinity throughout a receiving basin. Whereas nutrients and sediments are actively deposited and/or transformed as riverine derived water passes through an estuarine system, freshwater passes conservatively with no change, diluting higher salinity waters; (2) There is rapid and effective trapping of suspended sediments in estuarine systems due to decreased water velocity and sediment settling; (3) Nitrate concentrations are substantially reduced in the estuarine environment due to the processes of denitrification, assimilation and reduction; (4) dissolved inorganic molar Si:N ratios increase, and N:P ratios decrease as riverine water flows through an estuarine system.