Multi-channel patterns of bedrock rivers: An example from the central Narmada basin, India

An anomalous multiple channel pattern in bedrock is observed on a predominantly downcutting reach of the Narmada River. The multi-channel reach (800-2750 m in width and 8500 m in length) is bounded by major faults, and is underlain by granite and gneiss bedrock. Geomorpho-logical investigations reveal differences among the upstream, middle and downstream sub-reaches of the multi-channel study area. Whereas the upstream sub-reach is dominated by deep flows and fine sediments, the lower sub-reach is characterized by a steep gradient and rapids. The middle sub-reach is the widest, and is marked by thickly forested islands and boulder berms. The characteristics of the three different sub-reaches suggest control by the interactions of lithology, flood processes and tectonics. Estimations of Hack's (1973) stream-gradient index values indicate considerable variations for the SL values along the length of Narmada River. The highest value of gradient index (SL = 797) is associated with the multi-channel reach, implying lithologic or tectonic control. Given the dimensions of the reach and its channels, it appears that the present hydrological regime is inadequate to produce the feature. We hypothesize that the multi-channel pattern development in bedrock was initiated by block or domal uplift. Enhanced gradients and extreme floods permitted the system to exploit linear weaknesses in the bedrock, leading to the development of anabranches and establishment of multiple channels in bedrock. Abrupt changes in the channel planform and morphology at the study site indicate that the river is adjusting its channel geometry (width, depth, gradient and plan configuration) to a new equilibrium channel morphology through the action of the extreme floods characteristic of this fluvial environment.