Mardi Gras Pass

Mardi Gras Pass

Mardi Gras Pass is located in the Bohemia Spillway about 35 miles southeast of New Orleans at river mile 43.7. Mardi Gras Pass began to develop during the 2011 Mississippi River flood. In the course of a hydrologic survey conducted during the 2011 Mississippi River flood event, PC staff observed a process of overbank flow developing into a channelized flow across the crest of the natural levee. A breach was discovered in the roadway along the crest of the natural levee, which created a new channel. The breach continued to evolve through the natural forces of river flow. Headward erosion along the river across a forested bar allowed the channel to entirely breach to the Mississippi River in late February and early March 2012, at which time it was named Mardi Gras Pass. Mardi Gras Pass is now a free flowing distributary of the Mississippi River. Detailed observations and study of the pass through 2013 can be found in our report: Evolution of Mardi Gras Pass within the Bohemia Spillway of the Mississippi Delta in southeast Louisiana: March 2012 through December 2013 (email info@ScienceForOurCoast.org for a copy).

 

Location of Mardi Grass Pass within the Pontchartrain Basin. Mardi Gras Pass began to develop during the Mississippi River flood of 2011 when the breach in the road occurred. With water flowing over the natural levee, erosion continued until Mardi Gras Pass connected to the Mississippi River and became a free flowing distributary.

Why Study Mardi Gras Pass?

Mardi Gras Pass offers an opportunity to study the development of a new distributary of the Mississippi River over time. Because most of the Mississippi River is leveed, there is little opportunity for the development of new distributaries. The Bohemia Spillway, where the Pass is located, had its levees removed in 1926, which allowed for the development of Mardi Gras Pass over 80 years later. Additionally, Mardi Gras Pass allows us to observe the pattern of distribution of freshwater, sediment and nutrients into the basin from a new distributary. Lastly, it allows us to closely study natural processes that occurred prior to the construction of river levees.

PC Research Program

PC studies the changing channel dimensions caused by the water flowing through the pass from the Mississippi River. We also study the hydrologic changes that occur as the pass develops and channel dimensions change and how the hydrology and changing channel are related. The changing hydrology as the the pass develops leads to differing patterns of sediment and nutrient distribution into the basin which can support, sustain, nourish and build wetlands. Lastly, we look at what biological changes we see developing and assess what organisms are using the pass.

To measure the changing channel dimensions we use a high precision GPS to map the bank outline over time (left), connect the GPS to a depth meter to understand the channel depth and bathymetry (right). The breach in the road during the 2011 flood was the beginning of Mardi Gras Pass (middle). The picture on the right also shows where Mardi Gras Pass connects to the Mississippi River in the distance.

Results

So far we have reached a few major conclusions about the pass although we continue to study it. Mardi Gras Pass developed through a process of headward erosion, eroding from the marsh side toward the Mississippi River during high water and breached to the river in 2012. Discharge has increased over time for similar river stages due to channel enlargement which allows more water flow. The sediment and nutrient distribution from the pass into the basin results in the development of four zones; erosion close to the river where flow has a high velocity and is turbulent, sediment deposition when the water flow slows down upon reaching the larger waterway distribution system, a nutrient plume where much of the sediment has fallen out but nutrients still remain in the water column and a mixing zone where Mardi Gras Pass water mixes with bay water and returns to background salinity, nutrient and sediment conditions. Lastly, we know that many species of saltwater and freshwater fish are using the pass as well as mammals, birds, reptiles and crustaceans.

Example of the bank dimensions (left) and the channel depth (right) used to analyze the changing morphology of the Mardi Gras Pass Channel.

 

Many animals have been seen using Mardi Gras Pass. A otter sunning itself on the banks of Mardi Gras Pass (left), a spotted gar which is a freshwater fish (middle) and a speckled trout, a saltwater fish (right) all have been spotted in and around Mardi Gras Pass.

Resources

(email info@ScienceForOurCoast.org for a copy of a report or survey)

Presentations

Formation of Mardi Gras Pass
Governor's Advisory Committee February 2013 Presentation
Mardi Gras Pass Biological Assessment

Reports

Mardi Gras Pass Summary Jan 2013
Evolution of Mardi Gras Pass: March 2012- Dec 2013
Patterns of Sediment, Nutrient and Salinity Distribution from Mardi Gras Pass: 2013-2014
Mardi Gras Pass Regional Influence: Sediment Distribution and Deposition, Salinity and Nutrients in 2017
Lateral Flow from the Mississippi River at Mardi Gras Pass and Flow Distribution June through October, 2017

Fact Sheet

Mardi Gras Pass Fact Sheet

Bank and Bathymetry Surveys

April - May 2012
August 2012
September 2012
January 2013
March 2013
August 2013
January 2014
August 2014
February 2015
September 2015
July 2016
November 2016
August 2017
November 2017
July 2018
September 2019

Discharge Surveys

April 9, 2012
April 23, 2012
May 3, 2012
July 5, 2012
January 18, 2013
February 4, 2013
May 23, 2013
June 12, 2013
June 26, 2013
August 1, 2013
November 12, 2013
June 12, 2014
August 6, 2014
April 17, 2015
May 8, 2015
January 19, 2016
November 14, 2016
May 25, 2017
June 29, 2017
August 2, 2017