In my post on the Clean Water Rule and floodplain wetlands, I mentioned floodplain wetlands had been reduced so much after hurricane Katrina that scientists were concerned about the increased risk of flooding from future storms. I didn’t have time while I was writing the post to do a fact check on how much of the wetlands in Louisiana had been impacted, but I decided to follow-up just for my own information. I sat through a presentation by U.S. Geological Survey scientists twice on this subject, so I went looking to see if I could find the information on their website. I was amazed to find the very PowerPoint presentation I’d seen some eight years ago now.  It was produced by the Geological Survey’s Wetlands Research Center and the Louisiana Coastal Area Land Change Study Group. It provides a close look at land loss after the hurricane in comparison to land loss before the hurricane. They used satellite images to make their determination. I also found their final report on the subject: “Land Area Changes in Coastal Louisiana after Hurricanes Katrina and Rita.” (Here’s a link:http://pubs.usgs.gov/circ/1306/pdf/c1306_ch5_b.pdf). Bet you forgot about Rita, but yes there were two very destructive hurricanes affecting the Gulf Coast in 2005. The final report put the loss of wetlands from the two storms at a total of 217 square miles.

 

That sounds like a lot doesn’t it? But the reports from the Geological Survey put wetland loss from 1956 to 1978 in Louisiana at about 40 square miles a year. That’s an 880 square mile loss over 22 years. The Geological Survey also reports an approximately 35 square mile loss of wetlands per year from 1978 to 1990 for a 12 year loss of 420 square miles. Of course all these losses took years not hours to occur, but what is the reason for such incredible losses of wetlands?

 

People who are familiar with Louisiana and the great Mississippi River know the answer to that question. The Mississippi River isn’t really a river any more. It’s just a channel conducting water. The same goes for its major tributaries: the Missouri River, the Arkansas River, the Ohio River, and so on. Yes the mighty Mississippi, storied in American history and literature, is an over engineered navigational conduit to get goods from North to South and a pesky source of flooding to landowners along its length. The start of all the troubles began early, even before Mark Twain’s time as a steamboat captain. The steamboats went aground all the time in big piles of snagged trees in the middle of the river and on sandbars. So engineers devised ways to remove those snags and dredge the sand. But the real change for the Mississippi came after the flood of 1927, which was the beginning of the end for the free flowing great water body. I went back to the Army Corps of Engineers documents and also looked around at some of the National Research Council’s reports, but the best document I found on the channelization of the Mississippi came from a compilation report produced by the Geological Survey back in 2012. It is titled: “A Brief History and Summary of the Effects of River Engineering and Dams on the Mississippi River System and Delta.” (Here’s a link to the report: http://http://pubs.usgs.gov/circ/1375/C1375.pdf).  I like the Geological Survey’s reports. They are always well written, not like the mumbo jumbo you get out of some federal science agencies.

 

The report says that in 1927 about 43,500 square miles of land in 7 states was flooded and 246 people were killed. The flood of 1927 was kind of a legendary flood but most people today have never heard of it. All sorts of historic events are claimed to be a result of the flood, including the great migration of displaced African Americans from the Mississippi delta to northern cities and the election of Herbert Hoover as president of the United States based on his oversight of the flood relief efforts. (Presidents Hoover and Carter are the only engineers ever elected president). Basically a plan was implemented starting in 1929 to straighten out this flooding problem. And I do mean straighten out. Rivers are big complex bodies of water. They don’t have one channel, they have multiple channels. They meander over their floodplain. The basic mechanism creating meanders is high velocity water undercutting a river bank then depositing the sediment downstream on a bank on the other side of the river where the velocity of the water is lower. A program of cutoffs was instituted to reduce the number of channels and to cutoff large meanders. This program literally shortened the river. The lower Mississippi river lost some 145 miles as a result of this program. Then revetments were added to the banks of the Mississippi. Don’t know what a revetment is? They are essentially concrete mats that cover the entire bank of a river all the way to where it joins the submerged river bed.  Rip rap is also used. Back when the engineers were first seeking to control the river, they used a lot of rock and matting. The idea is to stop bank erosion and keep the river from migrating.

 

There were already levees before the time of the 1927 flood. Now there are miles of levees. You probably remember seeing them on television during the Katrina hurricane coverage. They are tall, wide compacted, earthen dam like structures that parallel a river. The Geological Survey says there are 3500 miles of levees just in the lower part of the Mississippi River Basin. There are levees everywhere in Louisiana on the banks of the Mississippi. In New Orleans you can stand on the ground in some areas and see the levees towering overhead with the river at the crest of the levee and a cargo boat steaming by above you. The levees are there to keep the river from flowing into its’ floodplain when water in the river is high. They also starve the floodplain of sediment and nutrients.

 

The other types of engineering control you’ll find on the Mississippi River are floodways and dikes. You’ll see floodways in New Orleans and other towns on the Mississippi River. They are there to redirect flood waters from the river away from a city into an area that is not quite as populated. They are big structures and the lower Mississippi in Louisiana has three really large ones. Dikes are just like the dikes you see sticking out into the ocean when you go to the beach, but on rivers they are usually protruding at an angle from the river bank. The purpose of the dikes is to direct the strong currents away from the bank to prevent erosion and meandering. What results is a strong current right down the middle of the river which scours out the river bed sediments and actually makes the river deeper.

 

One thing you won’t see on the Mississippi River thank goodness is a dam, at least in the lower part of the river. The upper Mississippi has a series of navigational locks and dams starting around Minneapolis that extend down to the Missouri River. All the main tributaries to the Mississippi have dams though. The Missouri river has six big ones. The Missouri River is essentially a system of reservoirs interspersed with channels. Again the dams provide flood control, but the big thing they prevent is sediment moving down into the Mississippi River. The Geological Survey says that the dams on the Missouri River have resulted in an enormous amount of sediment from reaching the Mississippi River and subsequently the floodplain wetlands in Louisiana. I didn’t realize that the Missouri River was the greatest contributor of sediment to the Mississippi from any of its tributaries. The Missouri drains the Great Plains and includes flow from the Arkansas and Red River Basins. Due to the semi-arid environment and lack of vegetative cover to stabilize soils, there is a lot of erosion. You would think with the dry climate there wouldn’t be a lot of transport of the sediment, but apparently rains during the fall are sufficient to wash those sediments into the Mississippi. Or at least that’s what used to happen. Now with all those dams, the sediment is just sitting there filling up the reservoir behind the dam and starving the downstream wetlands.

 

The Geological Survey report says that only about 40 % of the sediment that used to be transported by the Mississippi River makes its way to the Louisiana coast now. No wonder there is such a big loss of wetlands. Add in all those levees in south Louisiana preventing the river from connecting with the floodplain wetlands and you get essentially what I would call an ecological disaster in the making. Here’s something even worse though, although the Missouri River provides the lion share of sediment to the Mississippi River, it only contributes about 12 percent of the water. Half of the water going into the Mississippi comes from the Ohio River. Well of course, there is a lot more rain in Ohio, Illinois, Indiana, Kentucky, Pennsylvania and West Virginia through which the Ohio River flows than the states through which the poor old Missouri River flows (Montana, South and North Dakota, Nebraska, Kansas and Missouri).  But you know what exists along the Ohio River – lots of farms with soybeans and corn and lots of farms with cows and pigs and other animals. What with all the fertilizer and manure runoff, the Ohio is the largest contributor of nitrogen to the Mississippi River. If you have been reading my posts on the Clean Water Rule, you will know that wetlands are a great means of removing nitrogen from water. You disconnect the river from the floodplain and wetlands and all that nitrogen goes down and empties into the Gulf of Mexico. What happens next is famous – the Gulf of Mexico dead zone. All this nitrogen pouring into the Gulf results in a massive growth of phytoplankton which depletes the oxygen in the water and results in fish kills. The Geological Survey report said that in 2002 the dead zone in the Gulf of Mexico was as large as the state of Massachusetts.

 

So there you have the result of a river not being a river anymore. Dams exist on so many of our major rivers and have forever altered the landscapes downstream. I will be looking at the scientific literature on dams over the next few months and writing about the impacts to water quality and morphology of landforms.