Is the Mississippi river 1,000 feet deep, 100 feet, or do we really know?

People often ask that question as they gaze across the mighty Mississippi River. When DeSoto, as the first European, came upon the Mississippi river in 1541, it was only considered a mighty obstacle to traverse. Some of the indigenous tribes thought that terrible monsters lived in the river and would pounce upon unwary travelers.

To early European voyagers floating down the river in canoes, pirogues, keelboats, and flatboats, the maximum depths of the Mississippi River were unknown, but gigantic maelstroms or whirlpools would have been very intimidating. It had to be really deep!

As steamboats and later diesel towboats began plowing up and down the river, water depth became a key factor particularly at the crossings where the main channel travels from one side of the river to the other dropping sediment along the way.

Today, the U.S. Army Corps of Engineers (Corps) continues to survey and maintain water depths up and down the river to ensure that congressionally-approved minimum navigation depths are provided year around.

These minimum depths include 9 feet in the upper Mississippi River (from St. Paul, MN down to Cairo, IL at the confluence of the Ohio River) mostly maintained with locks and dams, 12 feet in the free-flowing lower Mississippi River from Cairo to Baton Rouge, LA, and a whopping 45 feet from Baton Rouge to the Gulf of Mexico to facilitate ocean-going cargo ships.

Construction is underway to increase maximum navigation depths to 50 feet in this lowermost reach of the river.

In reality, water depths vary throughout the year, sometimes fluctuating over 50 feet during major floods. They say the deepest part of the Mississippi River is near Algiers Point near New Orleans – over 200 feet. However, even around Vicksburg and Natchez, the Mississippi River can easily exceed 100 feet, particularly around the bendways and downstream of stone dikes poking out into the river like giant fishing poles.

Almost a thousand stone dikes have been constructed in the lower Mississippi River strategically placed to move water towards the main channel, or in river engineering terms, the thalweg, to maintain a self-scouring 9-to-12-foot navigation lane even at low water.

Did you also know that the bottom of the Mississippi River from Vicksburg to the mouth is below sea-level? Below New Orleans, salt water intrudes upstream during low river stages threatening freshwater intakes. Consequently, the Corps constructs a saltwater barrier to prevent saltwater intrusion.

So, there is no easy answer on the depths of the Mississippi River, except to say our largest river in North America, and the third largest in the world, carries a lot of water from its source in Lake Itasca, Minnesota to the Gulf of Mexico as it drains 41% of the continental United States.

Jack Killgore, Ph.D., is a resident of Vicksburg. Prior to retirement, he was a long-time employee of the U.S. Army Corps of Engineers (ERDC). He also serves as an enrichment speaker on the Viking Mississippi and teaches river science courses at Tulane University. He can be reached at kjkillgore@gmail.com.