The Evolution of a Tie Channel

Once cut off from the main stem of the river, oxbow lakes commonly remain connected to the active river through narrow self-formed channels which evolve at the time of lake formation. These channels, referred to here as “tie channels” (Blake and Ollier 1971), allow the regular exchange of water and sediment; flow enters the lake when river stage exceeds lake stage and drains from the lake as river stage drops. Remarkably, these channels rarely become plugged with sediment and remain active for hundreds to thousands of years. We estimate that 65% of the floodplain lakes on the lower Mississippi River remained connected to the river through tie channels prior to large scale alteration of its floodplain; today a handful remain and few are in an unaltered condition. How a tie channel or any channel created by a sediment-laden current entering still water forms and evolves is poorly understood. We undertook the study of a tie channel in the Raccourci Old River oxbow lake, Louisiana, USA (Figure 1) with the goal of characterizing its morphology and documenting its evolution in order to develop a model of channel formation. Critical to such a model is the identification of the processes that control the construction of levees, regulate channel width, and determine the rate of channel advancement. The Raccourci Old River (ROR) tie channel is uniquely suited to address these questions. In addition to being the largest known tie channel on the planet, it is probably the best documented. Regular hydrographic surveys of the channel date to the late 1800s, aerial photographs cover it since the 1940s, and airborne laser swath mapping (ALSM) was performed in 2003. On field visits to the site in 2002 and 2005, we conducted topographic and bathymetric surveys, and collected sediment samples and cores for particle size distribution analysis and optically stimulated luminescence (OSL) dating.