Engineers have manipulated the Lower Mississippi River for 200 years. They removed large “snags” of dead trees, branches and other “woody debris” that clogged navigation channels. They lined the banks, first with large mats made from willow trees, then with mats made from concrete, to keep them from eroding. They also shortened and straightened the channel to make the movement of barges easier and to speed the downstream movement of floodwaters. And they built earthen levees to keep the river from overflowing regularly onto farms and cities.
All this engineering of one of the world’s largest rivers has produced important benefits for people and economies. But it has changed the landscape for some of the river’s smallest – and most important – inhabitants. As a result, there is much less naturally occurring woody debris in the river system. Therefore, aquatic insects, an important food source for fish found in the river as well as birds, bats and other creatures, have much less habitat needed for their survival. The insects colonize woody debris, which also creates more cover for fish and more feeding habitat for fish, turtles and other species. Almost all game and non-game fishes in the river eat insects at some point in their life cycles. Aquatic insects also are important indicators of water quality.
Audrey Harrison wants to change that in small but strategic ways.
Harrison, an expert in aquatic insects, or macroinvertebrates, at the U.S. Army Engineer Research and Development Center (ERDC) in Vicksburg, Mississippi, is working with the Lower Mississippi River Conservation Committee (LMRCC) to test new methods of adding woody debris to the river in ways that don’t interfere with the management of the Mississippi for navigation. The project, with the blessing of the U.S. Army Corps of Engineers, will test the use of simple traps to create wood structures to be colonized by aquatic insects.
Aquatic insects such as stoneflies (above), mayflies and caddisflies are important food for fish.
The traps would be built using large wooden poles connected by steel wires in a “V” shape. Previously collected woody debris would be placed in the traps initially, and the traps would collect additional floating debris.
The first trap, with funding from the National Fish and Wildlife Foundation and The Nature Conservancy, is planned for a secondary channel at Prairie Point, near Helena, Arkansas. The LMRCC already has worked in the Prairie Point channel by putting holes, or “notches” in stone dikes, which has allowed more consistent water flow through the channel to benefit fish and other wildlife and improve access for anglers and paddlers. The LMRCC has completed nearly 30 such “dike-notching” projects on the lower river, improving flows along more than 100 miles of secondary channels, which are outside the main navigation channel.
Harrison (left) and her colleagues prepare to study insect colonization on wood and other material at Prairie Point.
Adding woody debris to rivers and streams is nothing new. It is often used to improve river habitats for insects and fish. Harrison’s approach, however, is new for the Lower Mississippi River.
Harrison and ERDC have studying the importance of woody debris by suspending baskets of wood, leaves and other material from buoys placed in the Prairie Point secondary channel. “To put some structure in the water column, and to see how many different organisms and how many of those organisms use this structure is absolutely amazing,” Harrison said. “This study is telling us that this river has so much life in it, and especially invertebrate life.”
Harrison and the LMRCC hope the first woody debris trap will be installed in 2022.