Bird’s Foot Delta, part two
Editor’s note: This concludes a two-part post on the Bird’s Foot Delta at the mouth of the Mississippi River (Figure 1a and 1b), written by one of its biggest fans, Dr. Andy Nyman (click here for Part one). Nyman’s post was written just prior to April 7, when the Associated Press reported that a crude oil spill by Exxon-Mobil operations had fouled about 20% of the Delta National Wildlife Refuge that occupies much of the area (Figure 2).
I asked Andy to write a paragraph on his thoughts on the likely effects of the spill on the Bird’s Foot (see Addendum below).
by Andy Nyman, PhD
Five years ago I couldn’t imagine saying this, but here it is: I’d be willing to sacrifice all of the publically-owned land in the entire Bird’s Foot Delta if that sacrifice were required to gain two new active deltas upstream, such as is projected in Figure 1b above, from the Louisiana Coastal Master Plan. Don’t expect the numerous private landowners there to agree with me, however.
Those ambitious plans for new delta lobes may never be acted upon but we’ve hastened the loss of existing wetlands in the Bird’s Foot Delta (some of which are privately owned) and foregone opportunities to let the river build new wetlands there.
Since at least the 1970s, a 45 foot deep navigation channel has been dredged across the bar in SW Pass, the main channel of the Mississippi River, and this sediment has been dumped at the head of Pass a Loutre, a major ‘toe’ of the Bird’s Foot. The Pass a Loutre Wildlife Management Area (Figure 3) described in part one of my post, is named for this river pass.
Dredging experts claim that releasing dredged sediments back into the main channel would reduce current velocity, causing more sediment to fall out – and breaking the navigation dredging budget. Most of my colleagues believe this claim.
Dredging experts have also claimed for decades that placing these sediments in Pass A Loutre does not slow water there, nor cause more sediment to fall out there. Few of us agree, but this claim is the basis for policy.
The river remains unfazed by these Jekyll-and-Hyde explanations. Pass a Loutre, the source of virtually all freshwater and sediment for its namesake WMA, is clogging up. As this eastward river distributary dies the Pass a Loutre WMA dies as a wetland.
A sand bar began to form in Pass a Loutre in the 1980s, contributing to the death of a crew boat worker while I was there. Vegetated islands appeared in the 2000s.
Pass a Loutre carries so little freshwater and sediment that its north fork is no longer usable for navigation as a route to the Gulf as it was in the 1980s and its south fork no longer exists. The distributary now carries so little water that in 2009 I forded its eastern opening to the Gulf of Mexico, which would have been unimaginable in the 1980s – and didn’t even get my knees wet.
The decision last year by the State of Louisiana to declare sediment dumping in the pass inconsistent with state policy was a step in the right direction but seems incomplete to me because it focused solely on the lost opportunity to pump the dredged sediments a few miles away and use them to create new wetlands. There is still no policy that recognizes previous and future losses of existing wetlands and previous and future reductions in natural wetland building in Pass a Loutre WMA, caused by choking the pass with dredge disposal.
The decision this year to close the West Bay Sediment Diversion project appears to be a step in the wrong direction. West Bay is the only large sediment diversion constructed to date; it also is the only one of any size built on the main channel of the Mississippi River. Diversions such as Caernarvon and Davis Pond are river water diversions intended to slow the loss of existing wetlands, rather than sediment diversions intended to build new landscape.
I’m certainly not an expert on delta building but references I’ve read* show that the area of wetland created is proportional to the size of the diversion project. Large projects require considerable time to work, however.
We’re talking decades. The largest artificial sediment diversion project to date is the Wax Lake Outlet that was dredged across a natural ridge in 1942 to expand the flood outlet capacity of the lower Atchafalaya River near Morgan City. The project was not intended to build land but after three decades new emergent landscape appeared in 1973 and the ‘artificial’ Wax Lake delta continues to expand. Much smaller sediment diversions on small channels throughout the Bird’s Foot Delta generally take at least 8 years to produce new wetlands.
Despite its recent construction in 2003, bathymetric surveys in 2009 showed that sediment accretion from the West Bay Sediment Diversion project had exceeded subsidence in parts of the project receiving area for the first time in almost a century. Nevertheless, the public was told that a lack of significant wetland accretion contributed to a decision to shut down this ‘youthful’ sediment diversion project.
The real reason this sediment diversion is being shut down is a stubborn, decades-old denial of the costs of deep-water navigation to Louisiana’s coastal wetlands. This denial has recently been accompanied by widespread misinformation on the impact of this sediment diversion in particular.
A study by the USCOE confirmed that shoaling in the anchorage area downstream of the West Bay Sediment Diversion predated construction of the diversion project. It was also estimated that the project contributed only 20 – 40 percent of the shoaling. Despite the minor contribution of the project to reduced draft in the anchorage area, limited restoration funds will be tapped to pay the entire cost to dredge the anchorage. The project will be closed before there has been sufficient time to adequately assess its performance.
The precedent set by this decision appears to doom large sediment diversions anywhere on the main stem of the Mississippi River. If navigation interests can trump sediment diversions in my favorite location, the Bird’s Foot Delta, I presume that such projects will be prohibited everywhere.
The decision to forego wetland building in the Bird’s Foot Delta and hasten the loss of existing wetlands would nevertheless be acceptable to me in exchange for large sediment wetland-building diversions constructed upstream.
Unfortunately, I believe that these decisions were made to save money in the short term. This is like foregoing automobile oil changes to save money; the short-term savings are a tiny fraction of the long-term costs. I would hope that these policies could be changed to protect the wetland building and sustaining capacity of the Bird’s Foot Delta until we are certain that new, large, sediment building diversions can be constructed further upstream.
Once again, a delta in the hand is worth two in a plan.
*Large scale, see: Coleman, J.M. 1988. Dynamic changes and the processes in the Mississippi river delta. Geological Society of America Bulletin 100:999-1015. Medium scale, see: J.T. Wells and J.M. Coleman. 1987. Wetland loss and the subdelta cycle. Estuarine Coastal and Shelf Science: 25:111-125. Small scale, see Boyer, M.E., J.O. Harris, and R.E. Turner. 1997. Constructed crevasses and land gain in the Mississippi River delta. Restoration Ecology. 5:85-92.
An estimated volume of 430 barrels of heavy crude oil was spilled on April 7 from a broken pipeline at Delta National Wildlife Refuge, which contains almost half of the marshes in the entire Bird’s Foot Delta (Pass a Loutre WMA occupies almost the other half). I was more worried during July of 2008 about fuel oil spilled at New Orleans reaching the Bird’s Foot Delta because refined products are so much more toxic than crude oil.
Most of the fouled vegetation will survive if clean-up personnel do not cut it, or cut it so that the remaning stems are tall enough to exceed water level during the next few months. Unfortunately, some areas will be so heavily fouled that vegetation must be removed, generally resulting in permanent wetland loss.
Even in areas where fouled vegetation can survive while oil naturally biodegrades, wildlife will become fouled for weeks to months. This is important because even crude oil is more toxic to animals than to plants. Thus, minimizing permanent wetland loss by relying on natural biodegradation also temporarily reduces populations of some wildlife.
Simply by walking or using marsh buggies, responders will push oil below the marsh surface where there is no oxygen and natural biodegradation is much, much slower. Thus it is possible to cause more harm than good by traveling through a large, moderately fouled area to reach a small heavily fouled area.
I encourage readers who see fouled vegetation during the months to come to appreciate the difficult decisions facing spill responders today.
Edited by Len Bahr, PhD (firstname.lastname@example.org)