EVIDENCE OF TECTONIC CONTROL ON RIVER PIRACY IN THE FOOTHILLS OF THE LESSER HIMALAYA, INDIA

The primary focus of this study is to investigate the neo-tectonic influence on dynamic drainage organization and related phenomena, particularly along the Main Boundary Thrust (MBT) in the Dehradun area of Uttarakhand, India. The selection of the Rispana River for the present study was prompted by its drainage through the tectonically active Lesser Himalayan region in the north and its foreland in the south, presenting an opportunity to test the response of river piracy to neo-tectonics. It is the zone where the MBT separates the Sivalik group of rocks from the Lesser Himalaya. The effects of the Holocene Tectonic moment are evident in the drainage pattern of the Doon Valley.

To understand the morphotectonic evolution and river piracy in the study area, a variety of methods were employed, including the use of large-scale map, a Guide map, CARTOSET data, field investigations, drainage analysis, and examination of fluvial landforms on Quaternary sediments. Regional geomorphic analysis relies on topographical changes to identify various neo-tectonic features and drainage anomalies.

As a result of a transverse fault, the Rispana River captured the Nalapani Rao near the crossing of the MBT. Prominent geomorphic evidence of stream capturing includes fault scarps, landslides, a braided and wide channel course of the captured stream, a narrow and asymmetrical course of the capturing stream, uphill-facing scarps, displacement of terraces, faults/lineaments, and other prominent features observed in the study area. Among the geomorphic evidence, various morphometric indexes such as width and height ratio, basin shape index, sinuosity index, gradient index, elongation ratio, and basin shape are also identified for the identification of river piracy.

River incision along the transverse fault, reactivation of the MBT in the late Quaternary period, and high precipitation were the primary factors contributing to the river piracy. This caused the Rispana River to shift towards the N-S trending transverse fault, leading to Nalapani Rao becoming the misfit channel from the MBT. The period between 43 to 11 Ka represents a phase of uplift, incision, and piracy, with the mismatch arising due to high sediment supply from the hinterland, river incision along the transverse fault, and the high transport capacity of sediment.

Keywords: Rispana River, River piracy, MBT, Neo-tectonic, Nalapani Rao