The Middle Mississippi Side Channels–Part 1

Jefferson Barracks Dike Field

The Old Man: Jefferson Barracks Dike Field

Monroe County, Illinois

In the Middle Mississippi between the Missouri and Ohio Rivers narrow side channels–places where fish breed and feed–separated its islands from the shore line. Lightly vegetated sand bars are perfect nesting grounds for least terns, which are endangered. The birds nest communally, but like their privacy and are territorial. On a bare sandbar they can keep an eye out for predators, be they winged or four-footed or even well-meaning two-footed visitors. The Jefferson Barracks Dike Field was the perfect candidate for a nesting ground for the least tern.

Until the St. Louis District of the U.S. Army Corps of Engineers waged war against side channels in the Middle Mississippi. Channel training devises, wing dams, directed sediment into the side channels, filling them. To keep the river from adopting side channels as the main channel, that is the navigation channel, the engineers built closing dams across the heads of side channels. Silt eroding off adjacent farmland washed into the side channels, closing them and welding them to the mainland.

In 1985 the U.S. Fish and Wildlife Service declared the least tern endangered in sought help from the Corps of Engineers in their recovery. The engineers began finding ways to introduce water into closed side channels and to open new side channels in dike fields.

South of Alton, Illinois the Mississippi is an open river, unencumbered by dams. Here, Congress requires the St. Louis District of the Corps of Engineers to maintain a nine-foot navigation channel. It does so with channel-training devices, wing dams or dikes, which speed up the current, direct it to the center of the river, and force the river to scour a deeper channel. The dikes catch sediment between them and build sandbars along the banks.

At Jefferson Barracks, at the southern end of the St. Louis Harbor, the river runs along the valley wall on the west side and along the American Bottom, floodplain, on the east. The navigation channel runs along the west bank. This stretch of river is wide and it became shallower after 1959. The Corps dredged constantly. In 1992 the engineers constructed a dike field of five rock wing dams, ranging between 800 and 1000 feet in length, with trails, which ran parallel to the flow, that ranged between 400 and 500 feet. The engineers notched each dike in one to three places and set barge anchors at the end of each trail. The dikes rose between thirteen and eighteen feet above Low Water Reference Plane. The sandbars caught between each pair of dikes accumulated to elevations between eight and thirteen feet above the Low Water Reference Plane. The higher the dike the higher the new land between them. And, the dikes here were high because this was a particularly troublesome stretch of the river.

Even after the Corps built the dikes in 1992, the dredge Potter had to dredge this section of the river, 2,245,000 cubic yards of clean sand over ten years at a cost of $3,600,000. And, dredging is not easy in the busy St. Louis Harbor, where barge companies maintain numerous fleeting areas throughout the river channel and were reluctant to stop operations to accommodate a dredge, nor are they willing to cease operations to accommodate the construction of a new dike.

The building of the dikes created a stretch of river where few fish swam. The height of the sandbars meant that they were dry most of the time. Vegetation took root and covered them, not a hospitable place for least terns.

High water seldom washed away the vegetation. When the engineers notched the dikes, they hoped to create side channels between the true bank of the river and the sandbar. They created small pools on the downstream side of the notches, but no side channels.

In 2001 engineers in the Hydrologic and Hydraulics Branch of the St. Louis District examined the possibility of modifying the Jefferson Barracks dikes to create side channels for fish and islands for terns. They also hoped to increase the depth of the adjacent navigation channel.

Using an aerial photograph, the engineers built a scale model of the dike field and studied various alternatives for scouring new side channels along the east bankline and creating aquatic depth and diversity for fishes, creating an island between the side channel and the navigation channel, and creating a reliable navigation channel next to the island. They tried raising the dikes; widening and narrowing the notches dike; increasing and decreasing the number of notches in each dike; increasing and decreasing the height of the notches over the Low Water Reference Plane; subtracting and adding dikes to the field and adding dikes to the opposite bank.

They tested each new configuration. Would it create a self-sustaining sidechannel? Would it create a high elevation island within the dike field? Would it increase the depth of the navigation channel? Of the fourteen configurations they tested, three filled the bill. One created a good side channel, but a small dike in the field would interfere with barge fleeting. A second created a good navigation channel, but the side channel would be too shallow. The third worked. The small dike was removed and therefore did not interfere with barge fleeting, but the notches created a continuous side channel between five and ten feet deep at low water for fish and a nicely isolated, 190-acre island for the terns.

Nesting Least Tern, U.S. Fish and Wildlife Image

The proposed work–raising the dikes in the field to seventeen feet above the Low Water Reference Plane; notching the existing dikes at the bankline; adding new seventeen foot rootless dikes, that is dikes not anchored to the bank and but start several hundred feed out from the bank, a notch; artificially dredging the new side channel to ten feet below low water; armoring a scour hole just upstream and downstream of one of the dikes–was never done. The EPA examined the project, found the sandbar contaminated by chemicals spewed into the river from a chemical plant upstream, and stopped the project until the contaminants could be cleaned up.[i]

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[i] Dawn M. Smith, David C. Gordon, Aron M. Rhoads, Robert D. Davinroy, “Sedimentation Study of the Middle Mississippi River at Jefferson Barracks, River Miles 176.0 to 166.0, Hydraulic Micro Investigation, “ U.S. Army Corps of Engineers, St. Louis District, Hydrologic and Hydraulics Branch, Applied Engineering Center, November 2001, http://www.mvs.usace.army.mil/eng-con/expertise/arec/Model%20Study%20Report%20PDFs/JB%20Bridge%20CD/JB-BridgeReport.pdf; Telephone conversation with Mike Rogers, U.S. Army Corps of Engineers, St. Louis District, River Engineering, November 7, 2007.