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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, LPBF 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.

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.

LPBF Research Program

LPBF 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.

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