150 years of changes of the Chandeleur barrier islands
A couple of weeks ago, Dave over at Geology News had a couple of posts about the impact of hurricanes on the landscape – first, there was a post about what barrier islands are and how they form and then a post showing amazing (and quite scary) before-and-after photos of Bolivar Peninsula in the wake of Ike. Hindered Settling went into some detail regarding the redistribution of sediment with similar before-and-after photos of Galveston.
The New York Times produced a great graphic showing the changes that the Chandeleur barrier islands, which are just northeast of the Mississippi Delta, have undergone in the last 150 years. As you can tell, they are shrinking quite rapidly.
Graphic from the New York Times; Data sources from the USGS, Univ. of New Orleans, Louisiana State Univ., and Louisina Dept. of Natural Resources
Click on the image above to see a bigger version and then go to the original New York Times article to learn more.
Barrier islands are dynamic depositional features that are constantly being constructed, molded, sculpted, and eroded over time scales that certainly matter to humans (decades to centuries). The combination of diminished sediment delivery from the river combined with subsidence and rising sea levels make these transient features vulnerable to storms.
–
UPDATE (10/9/2008): Geology.com has a nice summary of storm surge damage on the Gulf Coast from hurricane Ike.
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Clastic Detritus 
possibly stupid question:
Shouldn’t agriculture increase the sedimentary transport?
hmmm … good question … in what way?
I’m trying to think back to a presentation I saw last year by Montgomery (who wrote a pop-sci book called ‘Dirt’) about that very topic. I think the short answer is, perhaps unsatisfactorily, ‘it depends’.
Anyone else wanna chime in?
Bruce Wilkinson, formerly at Michigan, wrote several papers that attributed much of the high modern sed flux in river basins to agricultural influences (Geology paper at http://geology.geoscienceworld.org/cgi/content/abstract/33/3/161 and GSA bulletin paper at http://bulletin.geoscienceworld.org/cgi/content/abstract/119/1-2/140).
And of course, poor soil use practices in the US during the 1930’s and 40’s resulted in meters of denudation from fields, and resultant high sed yield into much of the Mississippi drainage.
But, like you said, Brian, it depends: there were a couple of papers a while back that used lake records to suggest that Incan and Pre-Incan farming practices actually reduced sed flux. I reckon it depends on the details of local impoundment and water routing in addition to sed production.
Thanks for the links Eric.
Regarding this very specific example of the Chandeleurs, the thing to remember is that those deposits are interpreted to largely be from an older delta lobe … that is, when the main branch of the river was delivering sediment to that location. It has since shifted (if I remember, about 2,000 yrs ago) to it’s current position. So, the point about less sediment delivery is also about *where* it is being delivered in addition to how much.
I could very well be mistaken on this delta-lobe history … I didn’t look it up … anyone feel free to correct me or add more info.
In this particular instance, isn’t the barrier island problem something to do with the way they’re trying to maintain the current river channel with levees, etc.? So all the sediment basically by-passes the wetlands/barrier islands and gets dumped further out in the Gulf of Mexico?
Chris … yes, all the modifications of the river morphology (e.g., removal of sinuous bends, bank stabilization, etc.) have had a huge impact on sediment delivery/distribution. I don’t have a reference at my fingertips regarding differences in volumes of sediment … I’ll try and dig something up.
There were a couple of good posters about the Chandeleurs at GSA on Wednesday. (Coastal process posters during the free beer.) I’ve lost my abstract cd (argh), and I don’t have the program with me. But I remember one study was by the USGS, and another study was by a woman from the University of… Virginia, I think?
As a delta progrades into open water it gradually loses efficiency (ie. slope) and will naturally seek a more efficient flow path. At this point the main channel of the river will divert to another course. A modern example is the Atchafalaya River. During the 50’s and 60’s it became apparent that if nothing was done that the Mississippi River would change course and favor the Atchafalaya. This spurred the construction of the Old River Structure to limit flow through the Atchafalaya to 30% or less. A consequence of this is that most of the sediment suspended in the river’s water is dumped off the continental shelf, doing no good for the subsiding and storm tattered wetlands along the coast.
The delta plain consists of several individual delta lobes that developed from 7ka to the present. The lobe that supplied sediment for the Chandeleur Islands was the St. Bernard delta which was active from about 4k to 2.5ka, at which point deposition shifted to the LaFourche lobe (think straight south of Baton Rouge). When the sediment supply is cut off, wave energy becomes the dominant force in the system and winnows the sediments, concentrating sand and carrying away clays and silts. The sand piles up and forms a barrier island. The Chandeleur Islands are in danger now because so much of their sand has been scattered onto the sea floor where wave energy is unable to round it up again and push it onto the shore. What you see in the latest pictures is the remaining marsh platforms standing in open water with all the sand removed. The marsh platforms are resistant to erosion because of their clay composition, but sand is easily carried away by storms. After hurricane Katrina in ’05 the islands did not look much different from after Ike, but the persistent scattering of sand will definitely have an effect on their long term existence.
swess … thanks much for the comment … so, the Chandeleurs were constructed primarily during the St. Bernard lobe, which ceases ~2.5 ka?
They primarily formed after the delta lobe was abandoned, at which point the wave energy overcame the influence of sediment deposition, so technically they developed from 2.5ka to present.
swess … ah yes, that makes sense … the delta lobe avulses and then the sediment is redistributed into the barrier islands.
thanks for sharing your expertise
Do you have a current blog? I am working on a research paper for a Geology college course. I am looking specifically for information on whether there are igneous rocks on the Barrier Islands off the Coast of Mississippi. I’m not finding any definitive information. Would you be able to direct me? Thank you, Angie
Angie, I recommend that you go to your college’s library (or local library) and find the appropriate geologic maps for that area. Whenever I want to know what kind of rocks are in a certain place, that’s what I do. If those libraries don’t have that information then you might consult your state’s geological survey. Good luck with the project.