The Bonnet Carré Spillway was constructed in 1931 to divert floodwater from the Mississippi River into Lake Pontchartrain, an oligohaline estuary near New Orleans, Louisiana. The spillway was opened from March 17 through April 18, 1997. Infaunal macroinvertebrate population, surface and bottom salinity, and dissolved oxygen data were examined to determine the effects of this diversion. These data were obtained from November 1996 through November 1998 from five sites on an east-to-west transect across Lake Pontchartrain. A community composed of oligohaline taxa persisted during the freshwater period, but changes in dominance, and to a lesser extent composition, occurred over time and among sites. There was a pronounced spatial effect related to the distance of the sites from the spillway and from tidal passes. Based on decreases in species diversity (H′), abundance, and the number of taxa, it appears that the opening had a deleterious impact on the benthos. The cause of these changes cannot be attributed with certainty to any one factor, but we speculate that they were the result of reduced salinity, cyanobacterial blooms, and hypoxia/anoxia. Although recovery occurred, based on overall changes in macroinvertebrate abundance and Rangia cuneata clam biomass, there was no evidence of increased benthic invertebrate productivity.