Alluvial river response to active tectonics in the Dehradun region, Northwest India: A case study of the Ganga and Yamuna rivers.

In environments characterised by active tectonics, it is widely accepted that river morphology will be affected by active fault displacement. For example, there is documented evidence of change in channel slope, channel width, channel braiding patterns, grain size distribution trends, and stream power in response to active faults. Therefore, river morphology can carry a measurable signature of tectonic activity. Furthermore, it can be hypothesised that fluvial systems are in fact more sensitive to local faulting than raw topographic expression. This would mean that young active faults will affect river morphology before they are expressed in the local topography. Therefore, detailed morphological measurements of rivers in tectonic settings could allow for an early detection of faulting which is not yet expressed in the landscape. The Dehradun region of the Northwest Himalayan foothills is an ideal test case for this hypothesis. In this area, the Ganges and Yamuna rivers flow across an active thrust fault system, which is not yet clearly visible in the landscape. Therefore, longitudinal river profiles of the Ganges and Yamuna rivers from the Lesser Himalaya mountain front to ~35 km into the Gangetic Plain were extracted from a 90 m Digital Elevation Model and hydraulic features, including width of channel belt, channel slope, and geomorphic characteristics, were collected from IRS 23.5 m satellite images. Stream power was calculated by using channel slope and monthly discharge data. These data were complemented by field measurements of bed material grain sizes at ~5 km intervals. All data were analysed for the ~80 km Ganges and Yamuna reaches flowing from the Main Boundary Thrust, through the alluvial Dehra Dun valley and across the suspected active Himalayan Frontal Thrust, and ~35 km out into the Indo-Gangetic foreland. The longitudinal profile, width of channel belt, channel slope, braiding relationship, pattern of stream power, and grain size distribution all indicate river response to active slip on the Himalayan Frontal Thrust. Most importantly, channel slope (0.063) increases in response to an uplifted bed, and elevation drop over the fault axis. The Yamuna channel slope shallows upstream of, and proximal to the HFT (~0.0025) and steepens across the fault axis to (~0.0035) in response to an uplifted bed and elevation drop. Width of channel belts narrow across the HFT in response to constriction by uplifted topography from ~1500 m upstream to ~500 m across the HFT. This is reflected in the braiding index as both rivers flow as one channel across the HFT and as many channels in the Dun and foreland. The grain size trend along the Ganges reflects downstream fining by selective entrainment throughout the entire study reach with no variation interpreted as having a high enough stream power to move all sediment. The Yamuna indicates downstream fining through the Dun valley, yet grain size increases immediately, upstream of the HFT. This is interpreted as being due to low stream power within the Mohand Anticline caused by a low discharge and shallowing channel slope. This study concludes that the Ganges and Yamuna rivers are responding to active tectonic uplift of the HFT in the Dehradun basin, Northwest India.