The level, which originates in Minnesota in the north and empties into the Gulf of Mexico in the south, fell for the second year in a row.
This is due to higher temperatures in the summer months and reduced rainfall.
The "drought-stricken" river, as described by Newsweek, has allowed salty seawater to creep north and is already affecting drinking water in parts of Louisiana's southernmost Blackmines County.
- New Orleans Mayor Latoya Canterel wrote on Facebook: "Today (Friday), in coordination with the Governor's Office, I signed the City of New Orleans Emergency Proclamation due to saltwater intrusion into the Mississippi River."
- "We will continue to work with our partners at the local and state levels, while closely monitoring the situation."
- She reassured residents that "there is no water supply affected outside Blackmines County with salty seawater."
Louisiana Governor John Edwards declared a state of emergency in July due to record-breaking temperatures that killed 16 people.
At a news conference last week, Keith Hinckley, head of the Diocese of Blackemines, said that "potable water has been distributed to about two thousand residents of an area where drinking water supply has been mixed with salt water."
In an update on Friday, the governor said Louisiana was days away from requesting a state of emergency from the federal government for the same reason: saltwater leaking into drinking water.
It is no longer just civilian authorities, requiring the intervention of the US armed forces.
The commander of the New Orleans Corps of Engineering office, Colin Jones, said 25 feet (7 meters) would be added to an underwater threshold built south of the city in July to slow saltwater intrusion.