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Posted 2/3/2015

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By Our Mississippi, December 14
Mississippi Valley Division

When most people think of the Mississippi River, the wide, flowing water usually comes to mind. But for nature, river engineers and scientists, and ultimately river projects and environmental habitat, it is truly the sediment that matters.

Mississippi River sediments range in size from gravel to sands to fine clays.  Moving along the river bottom or suspended in the water, these sediments are the building blocks for the river’s main channel, secondary channels, wildlife habitat, and delta and coastal development.  These sediments are provided by large and small tributaries carrying sediments from 31 states and two Canadian provinces.

Sediment load drives many of the natural processes that impact the river.  Sediments are critical for a variety of wildlife habitats, such as sandbars used by the interior least tern, an endangered species on the river.  River sediments are also one of key ingredients for coastal marsh restoration efforts on the Louisiana coast. 

The more we understand about sediments in the river, their distribution, particle sizes, historical data, and related information, the better we can plan and work with existing natural processes to minimize navigation and flood risk reduction project impacts while enhancing environmental habitat.

Sampling Now & Then

In the summer of 2014, the U.S. Army Corps of Engineers completed a comprehensive river sediment sampling effort. This sampling is building off similar sediment sampling efforts conducted in 1935 and again in 1989 and is greatly expanding the knowledge base of river sediments.

“We took 733 samples along the river from north of St. Louis to the Gulf of Mexico,” said Thad Pratt, a research physicist at the Corps' Engineer Research and Development Center in Vicksburg, Mississippi. Pratt managed the sampling collection and analysis effort.

“We replicated the 1935 and 1989 studies. We used the same drag sampler. And all three sampling efforts were done at about the same time of year and same approximate river stages. This will allow true apple to apple comparisons of the current and previous sampling data. We even talked to one living participant from the 1989 sampling, Dr. Fred Ogden of the University of Wyoming, who was a student at the time. He gave us great insight into how it was done.”

Boat crews took samples within a few feet of water to up to depths of 175 feet around Baton Rouge, Louisiana. Most samples were taken from locations at the middle of the river, but every 10 river miles, samples were collected from a complete cross-section of the river.  A field log recorded each sample location, date, time, and type of sample (coarse, fine, etc.).

Lab Analysis

In the laboratory at Vicksburg, each individual sample was then dried, separated by particle size, weighed and the data logged in. The smallest particles (less than 65 microns) were analyzed using a laser diffraction particle size apparatus, which provides a distribution function for the entire small particle sample portion. The larger size particles were prepared and also analyzed using the laser diffraction method to identify the size classes of material of the large particle sample.

“The full data set from this sampling will be compared to the 1935 and 1989 data for the entire lower river,” Pratt said. “This will give an idea of how the whole system is changing, both how sediment size changes over the years and what changes are a result of geomorphology of man's impacts.”

Sediment Knowledge Results

Why does all this matter?

“Sediments are important to the health and welfare of the river,” said Pratt. “Certain types of fish and invertebrates need certain sediments. If this sediment isn't being delivered, that fish or that invertebrate will not be living in that river reach.”

River sediment knowledge is increasingly being applied in environmental projects, such as the recent joint U.S. Fish and Wildlife Service and Corps of Engineers conservation plan for several endangered species on the Mississippi River. Sediment input is being used at the current Island 63 project area (near Helena, Arkansas) and will be incorporated in other environmental projects at various river locations, he said. 

System-wide knowledge will also benefit from the sampling effort as the sampling data is used in newly developed, advanced computer-driven numerical models. Such models will help address future river sediment issues and will also provide critical input to overall river operations and management.

For more: www.mvd.usace.army.mil/mrgp.