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Not Enough Sediment in Mississippi to Rebuild Louisiana Wetlands

Fragmenting Wetlands in the Barataria Basin at Port Sulphur, Louisiana

Fragmenting Wetlands in the Barataria Basin at Port Sulphur, Louisiana

The dream that we can make enough freshwater diversions from the Mississippi into the Barataria Basin to the west and Breton Sound to the east to reverse land loss is a fantasy.

A pair of geologists at Louisiana State University issued a report last week, noting that we have deprived the Mississippi River of the sediment necessary to counter the raise in sea level and rebuild the Louisiana coast.

The researchers have concluded that the 8,000 dams we have built in the Mississippi Basin are the culprits. Any sediment that may flow out of the uplands into the tributaries gets trapped behind the dams.

Lock and Dam 26, Alton, Illinois

Lock and Dam 26, Alton, Illinois

We built the 26 dams on the Upper Mississippi to turn it into a profitable navigation channel. All the sediment that comes out of the uplands is trapped behind the dams.  The Corps of Engineers must dredge the navigation channel constantly to maintain its 9-foot depth.

There are six on the Missouri, which flows through soft, erodible sedimentary rocks and supplied the Mississippi with 60% of its sediment before the construction of its dams in the 1950s.

Niobrara River, at Niobrara, Nebraska

Niobrara River, at Niobrara, Nebraska

Take the Niobrara River, which flows to the Missouri at Niobrara, Nebraska.  It heads neat Lusk, Wyoming and flows along the northern margin of the soft, erodible Sand Hills in Nebraska, turns north, and empties into the Missouri.

It is a river that somehow has not been dammed or channelized, but it has still been changed by the construction of the Gavins Point Dam downstream from its confluence with the Missouri. The dam turned the Missouri into a lake and raised the level of the Missouri 2.9 meters at the mouth of the Niobrara.

Note: When a fast moving stream meets a still body of water, it deposits its sediment in the still body of water and forms a delta. That’s what the Niobrara does with the sediment eroded from the Sand Hills when it meets the lake-like Missouri. This is the case with other tributaries of the Missouri. It is all retained behind the Missouri River dams.

Missouri River at Niobrara, Nebraska

Missouri River at Niobrara, Nebraska

Before the construction of the dam, the Missouri carried away all that sediment to the Mississippi, which delivered it to a still body of water: the Gulf of Mexico and built the Louisiana coast.

Now what sediment the Mississippi does carry to the Louisiana Coast does not get to it because we have built levees clear to Venice, Louisiana to prevent the river from flooding and depositing its sediment on the coast. Hence, we have to design freshwater diversions to deliver sediment to the wetlands, which are starving for lack of freshwater and sediment. But, now we are finding those won’t work.[1]


[1] Etheridge, F.G., Skelly, R.L., Bristow, C.S., “Avulsion and Crevassing in the sandy, braided Niobrara River complex response to base-belel rise and aggradation,”  In Fluvial Sedimentology by Norman Dwight Smith and John Rogers, found in Google Books http://books.google.com/books?id=7i_pWcmzRZ4C&pg=PA181&lpg=PA181&dq=Niobrara+Missouri+River&source=bl&ots=94teq3rvPZ&sig=_Ua9y4paapkbENbXSDPBU6r6rhQ&hl=en&ei=pS5KSoq_K4f-NZ-c-fIN&sa=X&oi=book_result&ct=result&resnum=6.

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6 Responses

  1. Most of folks I talk to have the impression that the Missouri, and the Lower Mississippi River, are muddy because they are polluted by humans. They are always surprised when I tell them that the river now carries 1/3 or less sediment than before channelization or dams. Supposedly, in its natural state, about 1/3 of the sediment in the river down in Missouri actually came from the Yellowstone River – which is a LONG way upstream, above 7 dams that trap that sediment.

    Another source of sediments that is missing is from the constant bank erosion as the Lower Missouri moved aimlessly across its valley, consuming bottomland, islands and constantly creating new channels. This would create pulses of extreme silt and sand loads during high water – and that has pretty much been stopped by the channelization and bank stabilization of the Lower Missouri.

  2. interesting material, where such topics do you find? I will often go

  3. […] is in a pickle. A year ago two Louisiana State University scientists published a paper that stated there is not enough sediment remaining in the water column when the […]

  4. […] came the dams on the headwaters of the tributaries, only a few at first–on the St. Francis in Arkansas and the Yazoo in Mississippi, but more […]

  5. […] year ago a pair of scientists at Louisiana State University published a paper that detailed the fact that most of the sediment the river carried at the […]

  6. […] And to repeat what I wrote almost two years ago when researchers at Louisiana State University released a paper that that there is not enough sediment in the Mississippi as it flows through the Louisiana Coastal marshes to restore them by diverting sediment to them. […]

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