From 2002, Louisiana dumped up to 1.8 million cubic yards of dredged mud near Horseshoe Bend, and the river turned it into a 35-hectare island

Every year, dredgers dig up mud from shipping channels on Louisiana’s Lower Atchafalaya River. In the 1990s, they deposited the sediment at disposal sites along the riverbanks, near a bend called Horseshoe Bend, just north of Morgan City. By 1999, those sites were nearly full, so engineers tried something different in 2002, according to a project record from the U.S. Army Corps of Engineers' Engineering With Nature initiative. Instead of hauling the dredged sediment away, they piled it upstream of a small, naturally forming island and let the current disperse it.

A problem with nowhere left to put the mud

The Corps' account of the project explains that when the old disposal sites became full, officials considered three options. They could convert the sites into dry upland areas, pump sediment through a long pipeline into the open waters of Atchafalaya Bay, or pile the material mid-river in a 350-acre tract adjacent to a small natural island, near the navigation channel. The Corps chose the third option on a trial basis, largely to see what impact it would have on sediment buildup farther downstream.


Letting the river do the building

Between 2002 and 2018, crews placed approximately 0.5 million to 1.8 million cubic yards of dredged material near the island every one to three years. Instead of sculpting the land by hand, the engineers allowed the river's current to disperse the mounded sediment on its own, a technique the Corps calls self-design. This cycle built a landmass of almost 35 hectares over a period of about 12 years; the actual island footprint that formed within the larger 350-acre placement zone officials had designated for the material. Placement continued after that too. Officials add that some 500,000 cubic yards were added in January 2020, and the island has continued to grow since then.

What grew there surprised researchers

Project monitoring has identified four distinct habitat types on Horseshoe Bend Island, ranging from forest to shrub-scrub to open-water aquatic beds, and more than 85 percent of the plant life on the island is native species. In a 2018 study published in the journal Integrated Environmental Assessment and Management, researchers at the Corps, led by Christy Foran and coauthors, sought to quantify the island’s wildlife and navigation benefits. The project’s own ecological assessment found greater plant species richness on the island than on a neighboring natural island and an older, traditionally built dredge-material wetland. The island also became a large and successful nesting area for wading birds. The island has more invertebrate life than nearby natural islands that do not have similar underwater sediment features.

Checking whether the soil pulls its weight too

It is not only plants and birds that benefited. In another study published in the journal Ecological Engineering, Jacob Berkowitz and coauthors tested the ability of the island’s soil to remove nitrate, a nutrient that can fuel harmful algae blooms downstream. The study found that Horseshoe Bend Island removed nitrate about as well as an older, more conventionally built wetland nearby, despite some differences in soil formation between sites. This finding suggested that the natural placement approach did not compromise the benefits to water quality.

A side benefit for shipping as well

As the landmass grew, it narrowed the river's cross-section at that point, speeding up water flow through the navigation channel, the Corps reports. Because the flow was faster, less sediment settled back into the channel, and crews had to dredge it less often. That same account also notes that the approach avoided the additional pipelines, equipment, and land rights that the other two disposal options would have required, which comparatively lowered costs

What other rivers can learn

Horseshoe Bend Island is also featured as a case study in the Corps' Engineering With Nature Atlas series. The record also notes that officials are now applying similar sediment-placement techniques at other sites along the Louisiana coast and in the Philadelphia district. The project is an example of how dredging, generally regarded as a routine maintenance task, can serve a dual purpose as a low-cost way to rebuild wetlands by working in harmony with a river's currents rather than fighting them.