CHANGING RIVER PATTERNS IN NORTHEASTERN LOUISIANA DURING THE HOLOCENE – INTERPLAY OF TECTONICS AND AGGRADATION

The Mississippi Valley, including northeastern Louisiana, has experienced many changes in river and drainage patterns during the aggradation of the valley in response to sea-level rise. At the same time, however, northeastern Louisiana has been experiencing ongoing tectonic events that have resulted in significant geomorphic deformations. In our investigation of the geomorphic development of this area, we have been trying to distinguish between the aggradational and tectonic signatures.

Two major stream capture events have occurred in this area during the late Holocene: the capture of Bayou Bartholomew to the central Ouachita valley and the capture of the central Ouachita River to the lower Bayou De Loutre valley. The capture of Bayou Bartholomew and the abandonment of the Bayou Bonne Idee (former the connection of the Bartholomew to Bayou Boeuf) does not appear to have a tectonic cause, but is rather due to aggradation of the Boeuf-Bartholomew valley. Aggradation of the bartholomew-Bouef valley eventually allowed floodwaters overtop the drainage divide with tributaries of the Ouachita. The steeper gradients of these tributaries caused the capture of the Bartholomew.

The capture of the Ouachita to the De Loutre valley breached a major tectonic barrier that had impounded a lake within the Ouachita valley. This uplift is now more than 10 m high, and the lake extended for more than 80 km upstream. Breaching of this barrier followed capture of the bartholomew, and may have been partially due to the resulting additional flow. In addition, however, the Bartholomew built a levee belt across an arm of the lake and down the outlet channel, Bayou Desiard, restricted flooding and raising floodwater levels on the lake. Subsequent tectonic movements within the former lake basin have caused several generations of channel adjustment. Additional and continuing changes to the river patterns can be seen throughout the region caused by the active tectonics and continuing aggradation.