Part 1: Wetlands Perceptions
Part 2: Riverine Processes
Part 3: Wetlands Values
"With walls alone, one could only build an absurdly elevated aqueduct. Resistance times the resistance distance amplified the force of nature." - John McPhee, The Control of Nature
In part 4 I talked about all the structures and projects that we humans have built in our folly filled efforts to tame the Mississippi. Mainly those are hundreds of miles of levees, multiple floodways, spillways, damming of distributaries, rerouting of the main channel... The list of what we have built and the trouble we have engineered for ourselves is lengthy.
Some of those mimic natural parts of an undisturbed river system. Levees are natural, but they're typically short and are formed from heavier sediments dropped first after flood water overtopped the natural channel. Then a natural levee is built. Distributaries are normal - like the Atchafalaya distributing water from the Mississippi. And floodways and spillways are designed to mimic that process in the absence of the real thing.
So we captured the mighty Mississippi, locked her in levees, blocked off any water escape routes, and thought everything would be fine.
It didn't exactly go to plan. The Mississippi has massively flooded many times since the 1850s.
Flooding is a natural process of rivers, and part of how the rivers built their floodplains and deltas in the first place. So of course, disrupting the natural processes would end up wreaking some havoc sooner or later.
The 1927 flood is sort of the benchmark by which we measure all Mississippi River flooding. If you look at the NOAA gauge at Baton Rouge, the record listed is 47.28ft which was set on 5/15/1927 (link). That year the river did remarkable things, and it "tore the valley apart" (John McPhee, The Control of Nature). Hundreds of crevasses (breaks in the levee) appeared in levees from Cairo to the Gulf of Mexico, hundreds of people were killed, 26000 square miles of land were covered by Mississippi River floodwaters, and New Orleans was saved from destruction and despair because some guys blew a hole in a levee to relieve pressure.
The flood of 1927 literally did tear the Mississippi River Valley apart. Yet, by water volume, it wasn't anywhere near to being a record amount of water.
But the river broke through it's confinements and spread out across the alluvial plain, just as nature intended. The power of a major river like that is difficult to harness and possibly impossible to contain forever.
The 1927 flood is what sparked the 1928 Flood Control Act, which ultimately resulted in much more aggressive measures to attempt to control the mighty Mississippi and it's raucous floodwaters. Levees were built higher, dams were built, new water distribution methods were developed, and the river channel was straightened and some loops were cut off. In Louisiana, the Bonnet Carre Spillway was built at the site of previous crevasses in the Bonnet Carre area. They also built the Old River Control Structure, the West Atchafalaya Floodway, the Morganza Spillway, and more structures upriver north of Louisiana.
I could write for days about the confinements and manmade methods of distributing water from Mississippi, but my main point is that all of these efforts to contain the river and then distribute water on our terms, leaves many opportunities for the river to fight back.
In 1927 there was such widespread flooding and destruction that a whole new era of management began. In 1973, a similar thing happened and shuttled in another new era of water management.
The 1973 flood was the turning point into maybe realizing that we don't have the river under control as much as we think we do. The flood was massive, and nearly tore down the Old River Control Structure (I talked about this structure in depth in Part 4 of this series).
After realizing that they had almost lost the structure and then would have lost control of the river (lost from the engineering perspective, anyway), the US Army Corps of Engineers had to regroup and rebuild and only narrowly survived the 1983 flood.
In my own lifetime, I've seen the Bonnet Carre being opened every more increasingly to offset flooding pressures on New Orleans. The Morganza Spillway was opened for the first time in 2011. Every time the river floods and the 30% that is diverted through to the Atchafalaya is also high, the Old River Control is tested again and again. The Caernarvon and Davis Pond Diversions also divert water from the Mississippi, but they provide a flux of freshwater to the surrounding marshes and are not primarily designed to alleviate flooding pressure on the levee system.
But there are areas of the river where natural processes are at work. In 1926, the manmade levees for a 10 mile (ish) stretch on the east side of the river were removed to create the Bohemia Spillway. Here, the river is free to move as it pleases, to overtop its own natural levee in times of high water and spill out into the adjacent marshes. Nature didn't exactly fight back to earn it's freedom in this location, but it's certainly taking advantage of the freedom to rebuild marsh and fill in/reclaim old canals.
In 2011, during a wild flood year, a breach in the natural levee of the Mississippi River in the Bohemia Spillway area developed. "Mardi Gras Pass developed from a breach in the road located at the levee crest in May 2011 to a fully connected distributary of the Mississippi River by March of 2012." (source)
Mardi Gras Pass and the Bohemia Spillway are operating as nature intended - water flows over the natural levee during high water, water disperses out the main channel and into the marsh via Mardi Grass Pass, and marshes are built/nourished/resupplied/maintained. It's a beautiful system, and one we are currently trying to replicate with our spillways, diversions, and floodways.
It's too late to reconnect the natural waterways that were formerly distributaries of the Mississippi River. Bayous Manchac, Plaquemine, and Lafourche [which is also an old Mississippi River channel from ~2000 years ago] are all dammed and disconnected from the river. None of those waterways could be connected to the river without disastrous consequences, surrounded as they are by houses, businesses, and other human activities.
It's totally possible that the Mississippi could break down other defenses, but time will tell. We're currently in an unprecedented river flood so it'll be interesting to see what happens as the snow melts and travels downstream. Ultimately, the river will win. Maybe not this year, maybe not for 100 years, but at some point, I expect the lower portion at least of the Mississippi River will flow freely again someday.
References:
NOAA Mississippi River real time gauge at Baton Rouge
The Control of Nature by John McPhee
Bonnet Carre Crevasse, Wikipedia
The flood of 1927 literally did tear the Mississippi River Valley apart. Yet, by water volume, it wasn't anywhere near to being a record amount of water.
But the river broke through it's confinements and spread out across the alluvial plain, just as nature intended. The power of a major river like that is difficult to harness and possibly impossible to contain forever.
The 1927 flood is what sparked the 1928 Flood Control Act, which ultimately resulted in much more aggressive measures to attempt to control the mighty Mississippi and it's raucous floodwaters. Levees were built higher, dams were built, new water distribution methods were developed, and the river channel was straightened and some loops were cut off. In Louisiana, the Bonnet Carre Spillway was built at the site of previous crevasses in the Bonnet Carre area. They also built the Old River Control Structure, the West Atchafalaya Floodway, the Morganza Spillway, and more structures upriver north of Louisiana.
I could write for days about the confinements and manmade methods of distributing water from Mississippi, but my main point is that all of these efforts to contain the river and then distribute water on our terms, leaves many opportunities for the river to fight back.
In 1927 there was such widespread flooding and destruction that a whole new era of management began. In 1973, a similar thing happened and shuttled in another new era of water management.
The 1973 flood was the turning point into maybe realizing that we don't have the river under control as much as we think we do. The flood was massive, and nearly tore down the Old River Control Structure (I talked about this structure in depth in Part 4 of this series).
After realizing that they had almost lost the structure and then would have lost control of the river (lost from the engineering perspective, anyway), the US Army Corps of Engineers had to regroup and rebuild and only narrowly survived the 1983 flood.
In my own lifetime, I've seen the Bonnet Carre being opened every more increasingly to offset flooding pressures on New Orleans. The Morganza Spillway was opened for the first time in 2011. Every time the river floods and the 30% that is diverted through to the Atchafalaya is also high, the Old River Control is tested again and again. The Caernarvon and Davis Pond Diversions also divert water from the Mississippi, but they provide a flux of freshwater to the surrounding marshes and are not primarily designed to alleviate flooding pressure on the levee system.
But there are areas of the river where natural processes are at work. In 1926, the manmade levees for a 10 mile (ish) stretch on the east side of the river were removed to create the Bohemia Spillway. Here, the river is free to move as it pleases, to overtop its own natural levee in times of high water and spill out into the adjacent marshes. Nature didn't exactly fight back to earn it's freedom in this location, but it's certainly taking advantage of the freedom to rebuild marsh and fill in/reclaim old canals.
In 2011, during a wild flood year, a breach in the natural levee of the Mississippi River in the Bohemia Spillway area developed. "Mardi Gras Pass developed from a breach in the road located at the levee crest in May 2011 to a fully connected distributary of the Mississippi River by March of 2012." (source)
Mardi Gras Pass and the Bohemia Spillway are operating as nature intended - water flows over the natural levee during high water, water disperses out the main channel and into the marsh via Mardi Grass Pass, and marshes are built/nourished/resupplied/maintained. It's a beautiful system, and one we are currently trying to replicate with our spillways, diversions, and floodways.
It's too late to reconnect the natural waterways that were formerly distributaries of the Mississippi River. Bayous Manchac, Plaquemine, and Lafourche [which is also an old Mississippi River channel from ~2000 years ago] are all dammed and disconnected from the river. None of those waterways could be connected to the river without disastrous consequences, surrounded as they are by houses, businesses, and other human activities.
It's totally possible that the Mississippi could break down other defenses, but time will tell. We're currently in an unprecedented river flood so it'll be interesting to see what happens as the snow melts and travels downstream. Ultimately, the river will win. Maybe not this year, maybe not for 100 years, but at some point, I expect the lower portion at least of the Mississippi River will flow freely again someday.
References:
NOAA Mississippi River real time gauge at Baton Rouge
The Control of Nature by John McPhee
Bonnet Carre Crevasse, Wikipedia
Bohemia Spillway, Lake Pontchartrain Basin Foundation
Mardi Gras Pass, Lake Pontchartrain Basin Foundation
Mardi Gras Pass Is Building Land. Here’s Why It’s Important. RESTORE Council
Mississippi River Anatomy, America's Wetlands Foundation
Evolution of Mardi Gras Pass within the Bohemia Spillway of the Mississippi Delta in Southeast Louisiana:March 2012 through December 2013, Lake Pontchartrain Basin Foundation
Mardi Gras Pass, Lake Pontchartrain Basin Foundation
Mardi Gras Pass Is Building Land. Here’s Why It’s Important. RESTORE Council
Mississippi River Anatomy, America's Wetlands Foundation
Evolution of Mardi Gras Pass within the Bohemia Spillway of the Mississippi Delta in Southeast Louisiana:March 2012 through December 2013, Lake Pontchartrain Basin Foundation
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