EVOLUTION AND DEATH OF A DISTRIBUTARY ON THE MISSISSIPPI RIVER FLOODPLAIN, TYRONZA, ARKANSAS

In large alluvial systems such as the Mississippi River, distributaries siphon discharge from the main channel. Most distributaries disperse water and sediment from the main channel onto the floodplain and never capture the entire flow to cause an avulsion. Larger distributaries may flow into a tributary or into the master channel further downstream. They may also bifurcate to form smaller channels that terminate on the floodplain.

Dead Timber Lake is a stagnant terminal distributary of the Tyronza River and is caught within an abandoned Mississippi River meander belt. Six cores through the 32-km long Dead Timber Lake, from the distributary node to the terminous, show that channel morphology and channel fill changes laterally. The channel morphology, channel pattern, and sediment package deposited by the distribuary varies depending on the distance from the node and the substrate. At its node, the straight Dead Timber channel occupies a Mississippi River abandoned channel and initially flowed into a shallow oxbow lake. Two meters of massive to poorly bedded brown silty clay to sandy loam filled the channel and overlie Mississippi River lacustrine sediment. Downstream, the Dead Timber system exits the abandoned channel and meanders across Mississippi River point-bar sand. The Dead Timber scroll bar is underlain by 3.5 m of fine sand. In the Dead Timber channel, up to 3.5 m of gray clay has accumulated with evidence of intermittent fill based on interbeds of more dark gray organic-rich clay within the gray clay. At the terminus of the Dead Timber system, sinuosity and channelization is absent and sheet flow follows a swale on the Mississippi River scrollbar. Deposition of 0.1 m of clay loam with very fine sand is followed by episodic clay accumulation with a buried A horizon.

The Dead Timber stream is presently inactive. Ponding to form the lake and the resultant tree mortality in the lake may be caused by surface deformation in the New Madrid seismic zone. Evidence for seismic events in the area includes abundant liquefaction features in the area and in the cores examined for this study and uplift across the St. Francis River to form the St. Francis Sunklands with leaning and traumatized baldcypress trees. Tectonic structures in the area that may be responsible for any surface deformation include the subsurface Blytheville Arch and the surface Bootheel and Blytheville faults.