Gulf of Mexico
Tristan Baurick, NOLA.com | The Jon Hendrickson of the Army Corps of Engineers stands in a boat near piles of dredged Mississippi River sediment on Crater Island, Wisconsin. (Photo by Tristan Baurick, NOLA.com | The Times-Picayune)

Dredging, dams and other river controls starve places downriver of much-needed sediment

CRATER ISLAND, Wis. — There’s a 1.5-million-ton pile of sand in Wisconsin that should be in Louisiana.

Instead of flowing through the Mississippi River and splaying across the delta, rebuilding Louisiana land lost to storms and erosion, this Wisconsin sand was trapped by a dam and then dug up and tossed into a heap known as Crater Island. One side is covered in trees. The other side, where the Army Corps of Engineers deposits fresh sand, looks like a little slice of Daytona Beach on the Upper Mississippi. Locals call it the “Crater Island Party Cove.”

“Our dredging numbers are going up,” said Jon Hendrickson, a corps hydraulic engineer, during a visit to Crater Island. “And we’ve got to put it some place, so this is one place we unload it. Now we can’t fit anymore.”

It’s an ongoing problem. The volume of sediment dredged to maintain shipping depths and clear out the build-up behind dams has the corps in constant search for new dumping grounds. Some of the sand goes to nearby river restoration projects or is hauled off for industrial uses, but supply far exceeds demand.

The corps has at least 13 “material placement sites” in Minnesota and Wisconsin, and dozens more down the length of river.

Between 5 million and 7.7 million tons of sediment is dredged from the Mississippi and the Missouri River, its longest tributary, each year. That’s about as much sand as Louisiana used on the $46 million restoration of Pelican Island, one of dozens of barrier islands that protect coastal marshes and buffer the state from storms. Without new infusions of sediment, these islands are rapidly fading away.

Why not ship the sediment down to Louisiana, where the state is so desperate it has begun mining sand from the bottom of the Gulf of Mexico?

“Too far,” Hendrickson said. “Economically, we couldn’t justify it.”

The Mississippi Delta was formed over the past 7,000 years with soil and sediment pulled in bits and pieces from across a large swath of the U.S. Much of this material drains into the Mississippi, flows to its mouth and spreads along the Louisiana coast. Marsh plants take root, encouraging more sediment to accumulate.

Before dredging and other efforts to control the river, more than 440 million tons of sediment found its way to the Louisiana coast each year. The latest assessments of sediment flows indicate that only half that much flows through the river now.

The river is literally being starved of sediment, but dredging is only one of the causes. Dams hold back vast quantities of sediment in deep reservoirs. The 29 dams stretching across the Mississippi are only a fraction of the more than 40,000 dams scattered throughout the river’s vast watershed, blocking the Mississippi-bound flow of sediment in Pennsylvania, Tennessee, New Mexico, North Dakota and more than 20 other states.

The Missouri River had been a primary source of sediment into the Mississippi. Originating in Montana, the Missouri was laden with fine grit, a byproduct of the slow erosion of the Rocky Mountains. But since 1950, the Missouri’s contribution of sediment to the Mississippi has been reduced by about 70 percent, due largely to dams.

Another source of sediment — the river’s own banks — is increasingly off-limits. Levees and revetments, the reinforcements lining the banks, prevent the Mississippi from scouring its sides or gouging out sections of its floodplain. Revetments alone have reduced bank caving, a major contributor to the river’s sediment load, by 90 percent.

This graphic shows a time series of annual suspended-sediment discharge of the Mississippi River at Tarbert Landing, Mississippi, and the rate of construction of engineered levees and bank revetments along the lower Mississippi River. (Sean McKeown-Young, Advance Local Graphics. Source: Meade and Moody, 2010)
This graphic shows a time series of annual suspended-sediment discharge of the Mississippi River at Tarbert Landing, Mississippi, and the rate of construction of engineered levees and bank revetments along the lower Mississippi River. (Sean McKeown-Young, Advance Local Graphics. Source: Meade and Moody, 2010)

According to Alisha Renfro, coastal scientist with the National Wildlife Federation, Louisiana would need to capture and retain more than half of the reduced sediment flows to keep up with the state’s current rate of land loss. Problem is, Louisiana’s extensive levee system funnels sediment to the river’s mouth, where it falls off the continental shelf, doing little good for the state’s disappearing coast.

Hundreds of millions of dollars have been poured into replacing Louisiana’s dwindling supply of sediment.

Since 1998, the state and federal governments have spent $817 million rebuilding Pelican and more than a dozen other barrier islands on the Louisiana coast. The largest project, the revival of Whiskey Island off Terrebonne Parish, required 15.8 million cubic yards of sand — enough to fill the Mercedes-Benz Superdome three times.

The island will help protect the coast from storms and marshes from erosion. But there’s a big catch: the state will have to do the same costly rebuild in about 20 years. Barrier island restoration doesn’t fix the underlying problem — that sediment isn’t going where it’s most needed.

This U.S. Army Corps of Engineers dredge operates in the Mississippi River near Wabasha, Minnesota. (Photo courtesy of USACE)
This U.S. Army Corps of Engineers dredge operates in the Mississippi River near Wabasha, Minnesota. (Photo courtesy of USACE)USACE


Sediment dredged from the Upper Mississippi may be far away, but the huge volumes dredged in Louisiana has great potential to rebuild lost sections of the coast. The state has for nearly a decade required large-scale dredge operations to place sediment in restoration projects or pay a fee. However, the largest dredger, the corps, is exempt from the state law. Most of the sediment the agency dredges is simply heaped on placement sites or poured into the Gulf.

Louisiana has for years tried to get the corps to put more of its dredged sediment to wiser use. The agency’s response is often the same: it doesn’t have money to transport the sediment to restoration areas. It has to be disposed of as cost-effectively as possible, and that often means dumping it near where it’s dredged.

The state is pinning its hopes on a unique, expensive and increasingly controversial solution for restoring its sediment deficit. Two sediment diversions, both planned in Plaquemines Parish and costing a combined $2 billion, would allow the river to flow through gaps in the East and West Bank levees and into Barataria Bay and Breton Sound.

Both water bodies have grown as the marshes retreated, losing about 700 square miles since levees were built in the 1930s. The Mid-Barataria diversion, the larger of the two, could deliver up to 3 million cubic yards of sediment per year, the equivalent of 250,000 dump truck loads. The diversion could build 30 to 50 square miles of new land over 50 years.

The diversions are not without critics. Communities in Barataria worry that added river flows could increase flood risk in areas already plagued by frequent tidal and storm surge flooding.

Floating cabins line the bank of the Mississippi River near Brownsville, Minnesota. (Photo by Tristan Baurick, NOLA.com | The Times-Picayune)
Floating cabins line the bank of the Mississippi River near Brownsville, Minnesota. (Photo by Tristan Baurick, NOLA.com | The Times-Picayune)Tristan Baurick, NOLA.com | The

While the goal of the diversion is to restore the bay, shrimpers and oyster harvesters have grown accustomed to its altered state. Barataria has become saltier, bringing shrimp deeper into the bay and closer to shrimp docks. Adding freshwater could push the shrimp farther out and into rougher waters. Oyster harvesters say the reintroduction of Mississippi River water could add pollutants and tip salinity levels against oysters. The added sediment may also cover and suffocate oysters across a wide area.

While the diversions are debated, new research is hinting that the state may have been overly optimistic about their land-building prowess. The USGS found that the state’s estimates of sediment flows were outdated and taken too far up the river. More recent sampling closer to the diversion sites indicates less sediment, meaning it could take more years to reach the state’s goals for land growth.

Scientists have also found that Louisiana’s land loss crisis doesn’t stop at the coast. A Louisiana State University study discovered last year that erosion and the declining sediment supply is making the seafloor off the Mississippi Delta steeper. The sloughing away of the seafloor could destabilize the dense web of oil pipelines off the coast, increasing pollution risks. Steeper slopes also tend to speed up storm surges, likely producing hurricanes that will hit New Orleans and other coastal communities harder.

For centuries, the delta had been spreading both above and below the water. Now it’s in a “stage of decline,” said LSU geologist Sam Bentley.

“It’s an epic reversal,” he said.

Next: The levee paradox -- When one community built higher levees to protect itself, flooding worsened for neighbors.

This series was made possible in part by a fellowship from the Institute for Journalism and Natural Resources.

See NOLA article . . .