During most months of a typical year, the Mississippi River's volume of water flow is sufficient to prevent salt water from the Gulf of Mexico from intruding upstream into the Mississippi River above Head of Passes. When the river's flow falls below a certain level, salt water may begin to move upriver from the gulf. The intrusion of salt water upstream into the Mississippi River is a naturally occuring periodic condition.
The river's bottom profile is deeper than the Gulf of Mexico water surface level up to about 15 miles downstream of Natchez, Mississippi. Denser salt water flows upstream along the bottom of the River underneath the less dense fresh river water. Salt water's upriver travel can ultimately affect municipal drinking water and industrial water supplies.