A Unique Remnant

Prior to Western settlement, the vast majority of the of the Mississippi River Alluvial Valley (MAV) was criss-crossed with numerous winding, braided channel sloughs, larger meandering secondary rivers (like the St. Francis), and a smattering of abandoned channels and oxbow lakes.  The dominant land cover was bottomland hardwood forests of oaks and other mast producing trees that were adapted to seasonal flooding.  Within the wetter areas were cypress-tupelo swamps with standing water most months of the year.  As permanent European settlers moved west across the Mississippi river, this area was recognized as a having high agricultural potential.  The prevailing view of swamps was very different than today's understanding of the habitat and ecological services they provide, as is summed up well in this article from the Brick and Clay Record printed in January of 1912.  The brick and clay industry was invested in the installation of drain tiles (subterranean clay pipes to drain agricultural lands).  The article is titled WAR DECLARED ON SWAMPLANDS, and here is an excerpt mentioning the St. Francis watershed.

This assault on the natural hydrology and native forests of the region has been the largest threat to the natural habitat in eastern Arkansas and south east Missouri.  Both water and tree were something to be conquered, to be "reclaimed" from a "worthless" state.  The widening and subsidence of the natural channel caused by the New Madrid Earthquakes in 1811-1812 that created the Sunken Lands proved too difficult to completely tame, and the area was instead walled in with massive levees.  This abandonment preserved a remnant of the bottomland hardwood forests and terraces that were once common.

Often when we look at hazards to a watershed, we look uphill.  Although there are no shortage of issues upstream in the watershed that put its future health at risk, this area is unique in that some risks lay downstream.  The construction of the floodway that is mentioned above in the early 1910's diverted a portion of the natural course of the river out of the Sunken Lands into an artificial channel.  A short earthen dam was constructed at the head of the new floodway, essentially creating a reservoir that contained the Sunken Lands above Marked Tree, Arkansas.  In 1926 water control structures were installed at the floodway outfall, and a lock and sluice way were installed at the outfall into the natural channel.  The old channel of the St. Francis was not completely abandoned so it could continue to collect water that fell on the surrounding area outside of the levees, and support commercial boat traffic.  In times of high flow, the majority of the downstream flow was directed down the floodway to prevent flooding along the old channel.  This created two outfall locations that had to be managed in conjunction to maintain water levels within the Sunken Lands.

The soils of the area are silt, clays, and fine sands deposited from alluvial activity of the St. Francis, and historically the Mississippi and Ohio rivers as they meandered through the area in pre-history.  These native soils were used to build the levees, starting as far back as 1893 when the St. Francis Levee District was created.  These soils are not the most stable when saturated, and the combination of two outfalls, and multiple structures, has proven a consistent threat to the maintenance of the water level in the Sunken Lands.  In the 1930's, the original sluice way and lock on south east side of the Sunken lands sustained damage in several successive floods, and was deemed damaged beyond repair during a flood in 1938 that washed out 90 feet of levee.  To the consternation of upstream inhabitants, that relied on an average of 750 lockings per month through the levee, the US Army Corp of Engineers (USACE) determined there was no way to design a robust water control structure that would allow boat traffic through the levee.  An innovative solution to lift the entire flow of the river over, instead of through, the levee was designed by the Memphis District of the USACE.  Three nine-foot diameter pipes were installed to siphon water from the Sunken Lands into the naturally channel of the St. Francis downstream.  These structures are still in use today, and are a highly regarded engineering solution to a specific problem, with no pumping costs, and minimal maintenance.

The dam across the floodway, and the outfalls through it, have not had the same longevity.  The dam has been subject to scouring and overtopping damage multiple times, and the outfalls have gone through several different designs, most recently reducing two floodway water control structures into one in the late 1970's.  The Arkansas Game and Fish Commission (AGFC) currently operates the outfall at the St. Francis Floodway, while the USACE still operates the siphons.  The two agencies have an operating agreement according to the Wildlife Management Area (WMA) Master Plan to maintain the water level in the Sunken Lands at 210 feet above sea level between February and October, and 212 feet above sea level during the waterfowl hunting season of November through January.  This plan approximates the natural cycles of water flow with a focus on creating additional wintering habitat for migrating waterfowl.  The surface area of the lake at the summer pool is approximately 3,122 acres, increasing to 5,474 acres at 212 foot elevation.  Efforts are made in the current management plan to keep the water level below 214 feet to prevent driving whitetail deer and wild turkey out of the WMA.  The lack of higher ground for retreat in flooding events is thought to be a significant hindrance to the management of these two species in particular, especially turkey, as these higher water levels often occur during the spring nesting season.

The most recent levee breach initially formed in 2008, and can be seen to the east of the outfall structure in this aerial image above. The repairs to this area were started several times between 2008 and 2017, each time ending in failure due to additional flooding events and shoddy workmanship.  The overflow structure was left open for more than two years starting in 2015 to prevent additional damage to the earthen dam.  During the summers, this left the lake area nothing more than a dry river bed with several shallow, braided channels.  In the winter, the water level could jump up as much as 10 feet after storm event, only to drop back out to a mudflat several days later.  The Google Earth before and after below shows the unmanaged water levels. After almost 10 years of ineffective repairs to the dam, the AGFC reached out the the USACE for assistance in closing the scour, and the dam collapse was finally closed in 2017.

Hopefully this last repair will last, but the modifications to the areas ability to hold water that resembles the pre-settlement condition have historically been a major threat to the continued health of this remnant sliver of habitat.

Additional threats to the habitat quality within the Sunken Lands are increased sedimentation from agriculture runoff that is slowly filling in the lake, invasive woody species (Callery pear Pyrus calleryana), and invasive animals (feral hogs and unwanted beaver activity).  Management of the areas is additionally challenged by mulitple private inholdings.  Over 100 private landowners still hold a smattering of properties within the Sunken Lands, preventing cohesive habitat management, and have generated numerous legal battles associated with the public access for recreation, navigability, and water level maintenance.