Mardi Gras Pass: A new diversion on the Mississippi River springs to life
By John Lopez, Ph.D., Lake Pontchartrain Basin Foundation
The Bohemia Spillway, located along the east bank of the Mississippi River two miles south of Pointe a la Hache, La., is a rare opportunity to observe the natural processes and potential benefits of the Mississippi River flow into the Louisiana wetlands. Because there is no artificial river levee to obstruct flow during high water, the river has been flowing into the adjacent wetlands for 85 years. In 2011, the Lake Pontchartrain Basin Foundation (LPBF) conducted a hydrologic study of how much water enters the spillway and where it flows during floods. It was over the course of this work that LPBF researchers made an unexpected discovery: a new channel was being cut by the flowing water from the Mississippi River.
As the 2011 flood waned, we began noticing this new channel, and in July, the channel made a dramatic breach into the nearby roadway. On Mardi Gras Day 2012 (Feb. 21), scientists noted that the channel had reached the bank of the Mississippi River and shortly after, a complete breach into the river occurred. With this milestone, the channel is now an extension of the Mississippi River that helps distribute the river flow through the new distributary channel.
At this time, the distributary flow through the newly-dubbed “Mardi Gras Pass” is small, estimated to be less than 1% of the river’s peak discharge (5,000 to 10,000 cubic feet per second). The channel is 30 to 40 feet wide near the river but deep enough to capture river flow continuously even under very low water. This new diversion was not manmade – it was the result of natural river forces seeking a shorter outlet to the sea.
It can be expected that Mardi Gras Pass will expand over time. The rate of enlargement is of great interest because this process has not been observed in modern times, and the concern is that the diversion may become too large. However, enlargement of the pass may be desirable, because just one mile away, the new draft Louisiana Coastal Master Plan recommends a large diversion of about 4% of the river’s peak flow (50,000 cubic feet per second). This new diversion is estimated to cost $220 million, so LPBF is encouraging the state and Army Corps of Engineers to consider Mardi Gras Pass as an alternative, since it may provide the same wetland benefits for a much smaller cost and much sooner than a constructed diversion.
Another exciting aspect of Mardi Gras Pass is the rapid emergence of the riverine ecology. When the channel was just a few weeks old, schools of fish were observed migrating up current toward the river. These pogy fish were feasting on the plant detritus being washed into the pass from the river. The influx of fish to the area attracted river otters, which have been commonly observed feeding in the pass. Additionally, beaver, heron and other critters have begun taking advantage of the bounty created by the river flow in Mardi Gras Pass.
Support for this research is provided by The McKnight Foundation, Environmental Defense Fund, The Walton Family Foundation, Surdna and The National Audubon Society. To learn more about LPBF and the Bohemia Spillway, please visit SaveOurLake.org (go to Coastal > Technical Reports > Bohemia Spillway Documentation).
- Video: Bohemia Rising: Exploring the Mississippi Delta in South Louisiana, (Lake Pontchartrain Basin Foundation).
- Resiliency of the Bohemia Spillway and the Evolution of Mardi Gras Pass, Southeast Louisiana, (Lake Pontchartrain Basin Foundation).