An Assessment of Diversions of the Mississippi River

The construction of artificial diversions of the Mississippi River channel must be placed in proper

historical context in order to fully appreciate their implications. They are quite simply the latest in a

series of human interventions into the natural processes of the river system. Beginning with the

construction of the jetties at the mouth of South Pass in 1875 to the damming of Bayou Lafourche in

1904 to the channel cutoff program that began in 1929 to the construction of Bonnet Carre Spillway in

1931 to the Old River Control Structure in 1962 and the construction of locks and dams in the upper

reaches of the river, each of our interventions into the workings of the natural system has either failed

outright to accomplish its intended goal or it has induced a set of unintended consequences whose

effects are arguably more significant and in the opposite direction than intended purpose of the project.

The Eads jetties and the subsequent dredging of the main channel of the lower reaches of the river

punctured the land-building mechanism of the delta. The mouth bars that were obstructing navigation

were also the fundamental mechanism by which the delta built land. By obstructing the flow of the river

during peak floods, and effectively forcing the bed load sediments of the river up and out of the channel,

the delta built land at the site of the new delta throughout the 18th and 19th centuries. Eads breach of

the mouth bar at South Pass had an effect similar to sticking a pin into an inflated ball there was

forever forward a preferential route for the principal sediment load of the river to follow into the Gulf.

The upstream confinement of sediment supply by the locks and dams, also constructed for the benefit

of navigation, only served to exacerbate the problem. Today even as we force a portion of the rivers

flow down it current channel, the lower delta is in a state that Paul Kemp has described as hydraulic

contraction.

The channel cutoff program implemented between 1929 and 1955 was initially successful in reducing

the length of the navigation channel of the river by 207 miles. Throughout this time and since then the

river has continued to meander and attempt to re-establish local reach gradients that would balance the

water and sediment loads, and by 1989 the channel was only 100 miles shorter than it had been in 1930

(Smith and Winkley, 1996). The Bonnet Carre Spillway has been opened nine times since its

construction. The deposits of sediment in the spillway between the river and Lake Pontchartrain are

often pointed to as an example of the capacity of diversions to build land. There are some significant

caveats to using Bonnet Carre as an example for planned sediment diversions. First, the spillway has

only been opened during peak flood events. The sediment that was deposited during those events has

been adequate to support an industry that supplies river sand for local construction and home use, but

the total area of deposition in the spillway is only about 1,500 acres. The spillway is north of the city of

New Orleans where studies have shown the suspended sediment load of the river to substantially higher

than it is south of the city. It is improbable that the volume of sediment deposited during peak floods at

Bonnet Carre would be duplicated at diversions further downriver. Even if it were, that volume would

not be significant enough to create any meaningful area of new land. Second, Allison, et.al. (2013)

documented the formation of formation of substantial shoaling in the river channel during the 2011

opening of the spillway. These shoals, which were as much as 20 feet tall, did not jeopardize navigation

on that reach of the river because of the depth of the channel and lack of any deep draft vessels using it.

Shoals of that magnitude in the reaches of the river south of New Orleans could be a significant

impediment to navigation. As Allison et.al. stated in their study:

the downstream shoaling inferred from the 2011 flood data has major implications for maintaining navigability of

deep draft vessels in the river

.Third, several openings of the Bonnet Carre have resulted in toxic algal blooms in the lake. This result

of introducing nutrient-loaded river water into brackish estuary ecosystems is a classic unintended

consequence of intervention into a natural system.

By far the biggest and most significant intervention into the natural processes of the Mississippi River to

date has been the construction of the Old River Control Structure. It is a diversion project, but contrary

to common conception, the purpose of the structure is to force 70% of the flow of the river into the

channel of the Mississippi River, and to allow 30% of the flow to follow its natural and preferred route to

the Gulf down the course of the Atchafalya River. There can be little doubt that the control structure

prevented what would have been the next natural avulsion of the river channel during the 1973 flood.

This is probably the single most important thing to remember about coastal Louisiana over the past 50

years. It is not correct to say that the levees are starving the wetland marshes of sediment relative to

what they would be receiving in a natural system. In a completely natural system the river would have

changed course, and for the past 42 years the wetlands of southeast Louisiana would have been

receiving exactly the same amount of sediment that they do today, which is effectively zero. The

unintended consequence of the Old River Control Structure was that while it preserved the navigational

corridor provided by the river, it interrupted the next phase in the natural delta cycle which has created

the coastal wetlands as we know them today. The reason that so many people refer to the cumulative

land area change in Louisiana as a crisis is the total magnitude of the change, but it is important to

remember that the coast has always been changing. Nature did not maintain land area by the continual

perpetuation of land created by any one delta. It maintained a balance of no net loss. In a totally

natural system the area of land being submerged in southeast Louisiana would be matched by the area

of new land created at the site of the new delta in Atchafalaya Bay. The changes occurring in the

wetlands of southeast Louisiana are overwhelmingly due to the natural processes of subsidence.

Subsidence has been a continuously active agent of change in south Louisiana for the past 100 million

years. South Louisiana has never not been subsiding during that time period. The river was able to build

new land at the site of its deltas because the sediment supply delivered by the river exceeded the

effects of subsidence, but as soon as that sediment supply was cut off by a change in course of the river,

subsidence took over as the dominant effect and the delta was pulled below the surface. David Frazier

defined 16 historical deltas of the Mississippi River that build the wetland marshes over the past 6,000

years. He numbered them in chronological order from 1 to 16. While all of these deltas contributed to

building land while they were at the surface, deltas 1,3,5,6,7,8,9 and 10 have all completely subsided

below the surface. Small portions of the original areas of deltas 2 and 4 remain at the surface in the

stable areas of western Terrebonne and St. Mary Parishes. The average age of the marshes at the

surface is less than 1,500 years old, and they are all actively subsiding. There is nothing that humans can

do to affect the rate of subsidence in the wetlands.

River diversions certainly will not be capable of offsetting the effects of subsidence. The diversion

projects envisioned by the master plan have three fundamental problems: 1) no diversion, natural or

artificial, can coexist with river navigation for extended periods of time, 2) the combination of the rates

of subsidence and sea level rise, and the sediment supply of the river make it highly improbable that

they will be capable of building any significant new land, and 3) there is very clear evidence that the

introduction by diversions of the same nutrient-loaded waters that cause the dead zone into the salt

marshes of the Breton Sound Basin is causing irreparable damage to the marsh.

All diversions that have formed significant new land have had a single unifying characteristic the

formation of channel obstructing shoals. In 1880 the newly formed Mississippi River Commission

submitted a report to the congress that gave an assessment of the river, and its vital role in

transportation. One of the principal issues addressed in this report was shoaling of the river channel

and its effect on navigation. The report drew a very clear relationship between openings in the banks of

the river (either as natural crevasses or manmade diversions) and shoaling in the channel:

"Shoals are found in the river immediately below crevasses, which is difficult to refer to any other cause that the

loss of current velocity which takes place below the crevasse. As a portion of the volume of the river is drawn off by

the crevasse when it is first made, it is impossible that the current below the crevasse can then be as rapid as it was

before the occurrence. Being less rapid, it unable to sustain the whole quantity of matter held in suspension by the

more rapid current above the outlet, and consequently its surplus sediment falls to the bottom below the crevasse."

Mississippi River Commission Preliminary Report, 1880

The same report specifically addressed a proposal to create a diversion of the river that would flow into

Lake Borgne. The intention of the proposed diversion was to provide an outlet for flood waters to

lessen the effects of flooding in the main river channel. The commission recommended against the

diversion of the basis that it, like other diversions of the river, would create shoaling in the river that

would obstruct navigation:

"Should an outlet be made to connect the river with Lake Borgne, results similar to those which have occurred at

Cubitt's Gap and The Jump must be confidently anticipated. Below Cubitt's Gap it is an undisputed fact that the

former depth of the river has been largely reduced since the gap occurred. Below The Jump it is not so definitely

known what the shoaling has been, as charts of previous soundings are not now available in any exist, but the

Light-House Board has placed two buoys a short distance below The Jump, to warn vessels away from the shoals

that exist there. It is not possible to make the proposed outlet into Lake Borgne without creating a shoal in the

river below it " Mississippi River Commission Preliminary Report, 1880

It is apparent that shoaling around the natural diversions of the river remained and issue throughout the

nineteenth and twentieth century. This Notice to Mariners was published by the Hydrographic Office

under the authority of the Secretary of the Navy:

A recent survey of the lower Mississippi River made by the U.S. Army Engineers shows that a shoal on which the

steamer J.I. Luckenbach grounded on December 18, 1907, has entirely disappeared, and that there is now a depth

of 8 fathoms at mean low water at that point. The removal of this shoal was probably largely due to the effect of

sills placed at Cubits Gap and Pass a Loutre to divert the flow of water into Southwest Pass The Master, Mate

and Pilot, June 1908

Measurements of river flow by the Corps of Engineers in the spring of 1989 showed that the percentage

of the rivers flow entering Cubits Gap had increased from 10% to 20% of the total flow of the river that

passed Venice just upstream. In the years prior to this measurement a crevasse in the banks of the river

just inside the Gap had formed, creating a natural diversion called Brandts Pass. It is clear that the

increase in the portion of the rivers flow entering Cubits Gap is directly related to the formation of the

Brandts Pass Crevasse, which allowed river water to flow into the adjacent marsh. The Brandts Pass

Crevasse Splay is the deposit of sediments that resulted from this natural diversion of the river, and is

often pointed to as an example of the ability of sediment diversions to build new land. A 2010 study of

the crevasse by the U.S. Geological Survey National Wetlands Research Center found that about 1200

acres of new marsh were created by the splay. The Corps of Engineers reported in 1990 that shoaling of

the main channel of the Mississippi River just downstream of Cubits Gap had occurred during the time

that the Brandts Pass crevasse was open, and that this shoaling had resulted in the grounding of the

Soviet vessel the Marshal Koniev, severely disrupting navigation for a period of weeks. The Corps

report recommended a dredging program to alleviate the effects of the shoaling. It is important to

recognize that channel shoals are not just a collateral effect of the diversions that build land. They are

an integral component of their ability to create land. The shoal is not simply created by the reduction in

the velocity of the rivers flow at the point of the diversion. The body of the shoal itself acts to divert

flow and sediment into the opening. As soon as the shoal is removed, the diversion loses whatever land

building capability it may have had.

This is the fundamental fallacy of drawing analogies between the Wax Lake Diversion near the mouth of

the Atchafalya River and the sediment diversions envisioned by the Master Plan. Wax Lake is a diversion

in name only. It was dug in the 1940s to divert flood waters away from Morgan City. There was no

original intention that it would serve as a sediment diversion. After its construction Wax Lake was left

alone, and it was allowed to transform into an approximation of a natural channel - which primarily

means that it was allowed to capture a portion of the bed load sediments of the Atchafalaya River, and

it was allowed to develop channel shoals at the point of diversion and to develop channel-obstructing

mouth bars at the point of outflow. These are the three essential elements in the deltaic land-building

process. Every crevasse or diversion of the river that has ever built any new land has these three

characteristics.

The large sediment diversions of the Master Plan are specifically designed not to capture bed load

sediments. If fact they necessarily guard against this occurrence, which is commonly called "thalweg

capture". Thalweg capture, which means the river changes course at the point of diversion, is the

nightmare scenario for sediment diversion. In theory it could be reversed, but it would be massively

expensive. To insure that the thalweg, or the lowest point of the channel, does not follow the course of

the diversion, it is not cut all the way to the depth of the river. Since the bed load travels along the

bottom like a glacier of sand, the diversion will not have access to bed load sediments in this

configuration. The purpose of the "inlet flume" in the published design diagrams for the Mid-Barataria

Diversion appears to be to try to stir up sediment into suspension to get it into the diversion channel.

The fundamental problem remains - this is not how nature builds land. Without the essential land-

building component of the bed load sediments and channel-obstructing shoals, it is virtually impossible

that sediment diversions will be capable of any meaningful creation of new land

The passage of the Coastal Wetlands Planning, Protection and Restoration Act (CWPPRA) in 1989

provided federal funding for projects that were designed to restore the marshes of south Louisiana.

Principal among those projects were freshwater diversions that were designed to reconnect the river to

the marsh. The stated objectives of projects such as the Caenarvon and the Davis Pond Diversions

were to divert freshwater were to provide freshwater and nutrients to the marsh to counter the effects

of saltwater intrusion and to restore marsh growth. Twenty four years later there are many that

question the effectiveness of these diversions to maintain the health of the marsh and to offset the rate

of wetlands loss in south Louisiana.

All of the original river diversions constructed under the CWPPRA were freshwater diversions, meaning

that they only drew from the very top layer of the rivers water. The upper layers of the river water

carry only a minor portion of its sediment load in suspension. It was soon realized that in order to offset

the rate of wetlands it would be necessary to attempt to divert significantly more mineral sediments

along with the water. In 2003 the West Bay Sediment Diversion, was opened with a deep cut into the

west bank of the river just upstream of Head of Passes, and across the river from Cubits Gap. The West

Bay Diversion produced many unexpected results in the early years of its operation. Instead of building

new land, the current of the river leaving the diversion actually eroded a portion of West Bay. There

was no emergent marsh created in the first few years of operation, and in 2010 a Corps of Engineers

report found that the diversion was responsible for shoaling of Pilottown Anchorage Area just

downstream of the opening in the bank. On September 8, 2009 the Coast Guard had reported that the

oil tanker Eagle Tucson ran aground at mile marker 3 on the Lower Mississippi, near Pilottown. Just like

the effects of the natural crevasses at Cubits Gap, Brandt Pass and The Jump, as reported in previous

years by the Corps, the alteration of the rivers current by the West Bay Diversion had resulted in

shoaling downstream. The Diversion was ordered closed in 2010 by a federal judge, and shortly

thereafter an agreement was reached to allow it to reopen if the Louisiana Coastal Restoration and

Protect Authority (LCRPA), which manages it, paid for the dredging of the Pilottown Anchorage.

The construction of the West Bay Sediment Diversion was intended as a demonstration project for the

effectiveness of sediment diversions. What it demonstrated is the inescapable conclusion that sediment

diversions are not capable of building significant new land. Alex Kolker conducted an exhaustive

evaluation of the West Bay Sediment Diversion and concluded:

These data indicate that rates of sediment accumulation could be as high as 3 cm/yr across West Bay, and higher

in locations where there is sediment focusing. These rates match or exceed the low-end estimates of regional

relative sea level rise (RSLR), but fall below the maximum estimates of regional RSLR. In order for land to build in

West Bay, sediment deposition must first infill the bay, where depths range from 0 to 3 m. Assuming a linear

balance between water depth, RSLR, and sediment deposition rates, it would appear unlikely that large areas of

new land would develop in West Bay over a time scale of less than a few decades.

The combined effects of subsidence and sea level rise are simply too great to allow for the creation of

new land by sediment diversions. The probability of success of the proposed Mid-Barataria and Lowe-

Barataria Diversions is made worse by the fact that they are directed into two concentrated hot spots of

high rates of subsidence due to the movement of faults.

It does not take an advanced scientific degree to recognize the negative effects of freshwater diversions

on the salt marshes. The U.S.G.S. Study of land loss associated with Hurricanes Katrina and Rita draw an

inescapable conclusion that there is a causal relationship between a concentrated area of wetlands loss

and the outflow of the Caenarvon Diversion. It would be simply foolish to refute this relationship.

Several researchers with advanced science degrees have offered very sound and reasonable

explanations for the concentration of land loss around the outflow of the diversion. Kearney, et.al.

summarized these explanations in a 2011 publication:

Ultimately, the scientific basis for river diversions needs to be more convincing before embarking on a strategy

that may result in marshes even less able to survive hurricanes. The evidence indicates that diversions not only fail

to conserve mature brackish and tidal freshwater marshes, but disrupt plant physiology in ways that endanger

individual plant vigor and overall marsh survival. In this regard, there is no better illustration than the Hypoxia

Zone of what high nitrogen levels can do to delicate nutrient balances evolved over millennia in nitrogenlean

ecosystems, and the daunting challenges for reversing that damage.

Diversions of the Mississippi River envisioned by the Master Plan are only the next step in a succession

of interventions into the natural processes of the river. As with all of their predecessors in this

succession it is very likely that they will fail to achieve their intended objective and/or create as set of

unintended consequences that create conditions that are arguably worse than if they had never been

constructed. The message to the proponents of diversions is clearly the same one you would give to

someone who found themselves in the bottom of a hole and unable to get out stop digging. We have

proven ourselves to be horribly inept stewards of the bountiful natural river system with which we were

blessed. Going forward we should consider a broader meaning to the concept of restoration. It should

not be about concrete and steel construction projects in the marsh, or dumping mud onto naturally

evolving marsh ecosystems or sand onto naturally migrating barrier islands. Restoration necessarily has

to mean restoring the entirety of the natural system to a state as close as possible to what it would have

been if we had never intervened, and removing our influences wherever possible.

Chris McLindon

chris_mclindon@att.net

504-756-2003

mailto:chris_mclindon@att.net