The longest Mississippi River flood on record happened in 2019, when the river stayed at flood stage for an unprecedented 245 days.1 The duration and volume of the flood led to an increase of 60 percent more water than average entering Louisiana’s coastal ecosystems, which had both positive and negative implications for wildlife and habitat.2
It’s important to remember that Louisiana is one of the most productive landscapes on the planet because of the river, not in spite of it. For thousands of years, the Mississippi River has been the lifeline of our state. Its fresh water and sediment are what built our landscape and created the bounty and diversity of our ecosystems.3
Several conditions contributed to this unprecedented flood event:
1. Record precipitation in the Mississippi River Drainage Basin, as high as 150-250 percent above normal, resulted in the wettest year recorded in the last 124 years.
2. At 245 days, the river was above flood stage for the longest period in recorded history, surpassing even the 1927 flood record of 152 days.
3. Nearly 210 trillion gallons of water flowed down the river since the start of 2019, which is 64% greater than the 10-year average.
4. For the first time in its nearly century-old existence, the Bonnet Carré Spillway was opened twice in consecutive years. The Corps made history again in May 2019 when they opened the structure twice in the same year to relieve pressure on levees and prevent devastating flooding to local communities.
5. The spillway was opened for an unprecedented length of time – 123 days.
While the unprecedented river flood resulted in a mix of impacts to wildlife and habitat across the coast, there have been many false claims and general misinformation circulating about the potential impacts of fresh water, nutrients and sediment on coastal ecosystems. Some claims even conflate the influx of fresh water from the highest river on record to sediment diversions, critical projects to restore Louisiana’s coast. This comparison is not accurate for a number of reasons.
Louisiana must make critical restoration decisions based on sound science. There is too much at stake to do otherwise. Let’s take a look at some of the claims that have been made about the high river and diversions and set the record straight.
Claim: The fresh water that inundated the Mississippi River Delta system is killing off everything and is polluted.
Reality: It’s true that a record-breaking high river in 2019 – in both quantity of fresh water and how long it continued to flow – meant that oysters and other non-swimming organisms in affected areas had high mortality and displaced other species, like brown shrimp. However, while there were substantial impacts to some saltwater species, these are expected not only to rebound over time, but to actually increase in productivity in years to come. Fresh water and nutrients from the river will increase the overall productivity of our coastal estuaries over many years following a flood event.4
But claims that fresh water is killing “everything” are patently false.
In the midst of the flood, in areas directly influenced by the river, freshwater species like catfish, redfish and crawfish were thriving, bountiful and healthy. The flood will also help reset the system – clearing out predators, like oyster drills, and allowing both commercial and recreational species to flourish in subsequent years.
On the issue of water quality, an assessment by the Environmental Protection Agency concluded that Mississippi River water quality is good. After analyzing water from Baton Rouge to Head of Passes, the EPA found that river water is suitable for public drinking water supply, swimming, commercial and recreational fishing and boating, as well as fish, shellfish and wildlife propagation.5 This is in direct contrast to nearby rivers, like the Tangipahoa, which is considered impaired for both fish and wildlife propagation and swimming.6 That doesn’t mean that we shouldn’t continue to work to reduce runoff throughout the river’s watershed that negatively impact water quality.
Claim: The opening of the Bonnet Carré Spillway to manage the 2019 flood negatively impacted the surrounding ecosystem by putting too much fresh water in the basin, and proponents of sediment diversions want to increase that amount.
Reality: With or without diversions, coastal Louisiana will continue to experience flooding from increased precipitation and runoff from upriver as it heads down the Mississippi River to the Gulf of Mexico.7 That flooding will likely necessitate using the Bonnet Carré Spillway for its intended purpose – a flood control structure meant to protect communities from impending water.
When a flood does occur, we can actually use sediment diversions to help us reduce the need to operate the Bonnet Carré Spillway, distributing fresh water to areas where it can help with coastal restoration instead of dumping precious sediment into the lake or Gulf. That’s a major win-win!
Claim: Operating sediment diversions is the same as opening the Bonnet Carré Spillway.
Reality: Sediment diversions are operated differently from the spillway and are used for different purposes. The distinction between these two projects is very important.
About the Bonnet Carré Spillway:
The spillway is a flood control structure that was built and is operated to maintain the navigational system and protect communities in New Orleans, St. Bernard and Plaquemines parishes from devastating flooding. The spillway shoots Mississippi River water through Lake Pontchartrain and into Mississippi Sound and Lake Borgne, all large, open water bodies. The spillway is operated when the river hits a specific threshold no matter what the ecological impacts may be.8
About Sediment Diversions:
Diversions are coastal restoration projects, designed and operated to build land. Diversions are flexible restoration projects. They can maximize sediment capture at key times , but can also be turned off if large-scale impacts to fisheries are anticipated and determined to be caused solely by diversions.
Claim: This unprecedented flood and the opening of the Bonnet Carré Spillway put too many nutrients into the system, and sediment diversions will do the same.
Reality: Diversions allow us to put the nutrients where they’re needed and reduce them where they’re not.
Excess nutrients in river water come from the overuse of fertilizer on the wide expanse of agricultural fields in the entire Mississippi River watershed. The nutrients in open water can form algal blooms that, in warm weather and low winds, can lead to low-oxygen concentrations in the water.9 These conditions existed this year when high nutrients were introduced into Lake Pontchartrain from the Bonnet Carré Spillway leading to the development of algal blooms.10
Sediment diversions, by contrast, are specifically sited to empty into degraded wetlands. Just as our agricultural crops need nutrients to flourish, our wetland plants can utilize the nutrients to grow and become healthier.11 This also can remove a substantial amount of nutrients from the water before it gets to open water bodies, thereby reducing the likelihood of low-oxygen areas.12
Claim: We don’t have enough time or sediment in the river for diversions to work.
Reality: Each year, 90 million tons of sediment pass by Belle Chasse. Our problem isn’t a lack of sediment. It’s that the sediment isn’t going where it’s most needed to have the greatest land-building impact.13
There is less sediment available than there has been historically, but that doesn’t mean there isn’t enough. On the contrary, the available sediment supply is still huge and adequate to the challenge of sustaining targeted regions of coastal Louisiana.
By far, the most efficient way to utilize the sediment we do have is with sediment diversions. Seventy-five percent of the sediment the river carries is mud that isn’t deposited in the river channel and, therefore, it can’t be dredged.13 Sediment diversions capture the most mud and sand of any restoration tool. The state’s coastal wetlands can neither be rebuilt nor sustained, over the long term, without sediment diversions.
Claim: Restore the Mississippi River Delta and other environmental advocacy groups only care about diversions.
Reality: Restore the Mississippi River Delta supports 17 priority projects, most of which are not diversions.
Diversions are essential to coastal restoration because they are the restoration tool that will hang onto the most coast for the longest time. They are cornerstone projects because without restoring the natural function of the delta, we’ll continue to get what we have today – a coast that is disappearing – even in areas where other restoration projects occur. The freshwater and sediment from diversions are needed to sustain projects like dredge based marsh restoration and shoreline protection.14
We also recognize that how diversions are operated is key to their success. We are completely committed to finding the best path forward that gives us the most land-building while considering and maintaining other ecological functions. We rely not just on science, but on partnerships with fishermen, landowners, and others to inform our recommendations.
We can manage the system to maximize sediment delivery and the land-building power of diversions, while maintaining incredibly healthy fisheries and growing our fisheries to be more diverse.
The river built the productive wetlands of the Mississippi River Delta, and our only hope for continuing to survive in the face of hurricanes, saltwater intrusion and sea level rise is using the river to again build and maintain land. In fact, the only places gaining land in Louisiana are doing so because of regular input from the Mississippi River.
The Bonnet Carré Spillway is a flood control structure that is used in times of emergency to prevent closure of the navigation channel and catastrophic flooding to communities. In contrast, sediment diversions are flexible restoration projects that can build land and also maintain healthy and diverse fisheries. Decades of scientific research and coastal plans support diversions as critical to the future of our entire region.
We hope this helps bring clarity about the impacts of high river events like the 2019 flood and the importance of sediment diversions to the restoration of the Mississippi River Delta. We strongly believe in making decisions based on sound science and we also believe it’s critical to have an informed and engaged public in this restoration process on which our future depends. Please contact us for questions about diversions and opportunities to advocate for restoration.
4 Day, J.W., R.R. Lane, C.F. D’Elia, A.R.H. Wiegman, J.S. Rutherford, G.P. Shaffer, C.G. Brantley, G.P. Kemp. 2016. Large infrequently operated river diversions for MIssissippi delta restoration. Estuarine, coastal and shelf science. 183, 292-303.
5 2016 EPA 2016 Waterbody Report for Mississippi River Passes-Head of Passes to Mouth of Passes: Read here.
6 2016 Waterbody Report for Tangipahoa River-From Interstate 12 to Lake Pontchartrain: https://ofmpub.epa.gov/waters10/attains_waterbody.control?p_au_id=LA040702_00&p_cycle=2016&p_state=LA&p_report_type=
7 Frederick, B. 2019. Precipitation Trends in the Mississippi River Watershed: https://www.mvd.usace.army.mil/Portals/52/docs/Precipitation%20Trends.pdf
11 Morris, J.T., J.A. Nyman and G.P. Shaffer. The influence of nutrients on the coastal wetlands of the Mississippi Delta, In: Perspectives on the Restoration of the Mississippi River Delta: The Once and Future Delta, eds. J.W. Day, G.P. Kemp, A.M. Freeman and D.P. Muth, pp. 111-124.
12 William J. Mitsch, John W. Day, J. Wendell Gilliam, Peter M. Groffman, Donald L. Hey, Gyles W. Randall, Naiming Wang, Reducing Nitrogen Loading to the Gulf of Mexico from the Mississippi River Basin: Strategies to Counter a Persistent Ecological Problem: Ecotechnology—the use of natural ecosystems to solve environmental problems—should be a part of efforts to shrink the zone of hypoxia in the Gulf of Mexico, BioScience, Volume 51, Issue 5, May 2001, pp 373–388
13 Allison, M. A., et al. (2012), A water and sediment budget for the lower Mississippi–Atchafalaya River in flood years 2008–2010: Implications for sediment discharge to the oceans and coastal restoration in Louisiana, J. Hydrol., 432–433, 84–97.
14 Louisiana Coastal Protection and Restoration Authorities’ Louisiana’s Comprehensive Coastal Master Plan for a Sustainable Coast: http://coastal.la.gov/our-plan/2017-coastal-master-plan/.