Why Mississippi Sound salinity levels are low and how opening Morganza Spillway can lower freshwater intrusion

Mississippi Sound Salinity Level Update June 2, 2019

BILOXI, Miss. (WLOX) -As freshwater intrudes the Mississippi Sound, over one hundred dolphins and sea turtles have washed-up dead-on Mississippi beaches in 2019.

According to the University of South Mississippi Department of Marine Sciences, salinity levels in the Mississippi Sound vary from day to day and season to season. But typically, in the summer months, salinity levels are commonly in the 18-22 parts per thousand (ppth) range.

Salinity levels are nowhere near that.

As of early June, many readings in the Mississippi Sound were in the 0-5 ppth range.

How we got so low

The combination of locally heavy rain this winter and spring and the opening of the Bonnet Carre Spillway has led to abnormally low salinity levels in the Mississippi Sound.

The Bonnet Carre Spillway was built to release water from the Mississippi River into Lake Pontchartrain to reduce stress on the Mississippi River levee system and prevent cities like New Orleans from flooding.

When the Bonnet Carre was opened for the first time this year on February 27, 2019, it dumped hundreds of millions of gallons of freshwater daily into Lake Pontchartrain. That went on through the second week of April. Salinity levels began to decline in the Mississippi Sound.

Sediment from Mississippi River floodwaters can be seen moving into Lake Pontchartrain and the Mississippi Sound as water flows out of the Bonnet Carre Spillway
Sediment from Mississippi River floodwaters can be seen moving into Lake Pontchartrain and the Mississippi Sound as water flows out of the Bonnet Carre Spillway (Source: MODIS CIMMS University of Wisconsin)

But, heavy rain in April and May lead to the Pearl, Wolf, and Pascagoula Rivers flooding, increasing the amount of freshwater being dumped directly into the Mississippi Sound. This resulted in salinity levels tanking.

Radar estimates of over a foot of rain fell in parts of Pearl River and Stone counties in one weekend.
Radar estimates of over a foot of rain fell in parts of Pearl River and Stone counties in one weekend.

Unrelenting rains further north lead to major flooding in Oklahoma, Arkansas, Missouri and the Ohio River Valley. These areas all drain into the Mississippi River, prompting an unprecedented second opening of the Bonnet Carre Spillway and is expected to remain open until further notice.

The Morganza Spillway

How the Morganza Spillway can help lower freshwater intrusion in MS sound

Water always tries to find the path of least resistance. This is true for the Mississippi River. The easiest path it can take to the Gulf of Mexico is by connecting with the Atchafalaya River.

However the Army Corps of Engineers built a levee system and structure called the Old River Control Structure that keeps the Mississippi River on its course through major port cities like Baton Rouge and New Orleans and prevents the Atchafalaya River from taking over the Mississippi River.

The Morganza Spillway is located northwest of Baton Rouge and is meant to release water from the Mississippi River and take pressure off of levees when the water is too high.

The floods of 2019 have been so high and persistent, the Army Corps of Engineers will open it in early June to relieve pressure off levees to prevent cities like Baton Rouge from flooding.

The opening of the Morganza Spillway floods the Atchafalaya River basin which is the largest area of wetlands in the United States.

Unfortunately, there are towns and farmland that flood as well, and emergency officials are making sure people are out of harms way.

How opening the Morganza Spillway lowers freshwater intrusion in the Mississippi Sound

By opening the Morganza Spillway, water is diverted from the Mississippi River into the Atchafalaya River Basin, this water flows down directly into the Gulf of Mexico.

In contrast, water that flows out of the Bonnet Carre Spillway flows into Lake Pontchartrain then into the Mississippi Sound.

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