Late Quaternary Geomorphic Evolution of the Lower Subarnarekha River Basin: Palaeoclimatic and Hydrologic Implications

Fluvial systems are repositories of past climatic and hydrological conditions, being sensitive to climatic, tectonic and environmental perturbations. Changes in these conditions trigger cascading effects within the system, altering the channel behaviour/morphology, geomorphic form and hydrological regime. Such perturbations often get preserved in sedimentary sequences and provide important links for understanding palaeo-environmental conditions. These links aid in reconstructing the evolutionary history of the fluvial system and their response to climatic and hydrological changes. The present work investigates the chronological evolution of the Subarnarekha River basin in eastern India. Situated within one of the world’s oldest cratons, this river’s corridor contains several remnant incised meander cutoffs on its upper terrace surface, steep successive terrace flanks, lateral meander migration signatures and well-defined scroll bar sequences on the adjacent floodplain. These topographic signatures denote possible alterations in the Subarnarekha River’s palaeo-hydrological regime, and its concomitant response. In thus study, such terrace deposits and palaeo-meander scroll bars have been investigated using high resolution DTM (2.5 m), geophysical imaging (GPR), sediment textural analysis and optically stimulated luminescence (OSL) dating techniques, along with historical maps, to reconstruct the evolutionary trajectory of this river and its possible causative factors behind the discerned channel alterations. The obtained geochronological results reveal traces of sequential floodplain reworking during the Late Holocene. The Subarnarekha’s paleo-meander hydraulic parameters were ascertained to be much higher compared to the present-day active channel parameters, and contains multiple sinuous scrolls along the adjacent floodplain. Bankfull paleo-discharge was estimated using channel dimensions of remnant cutoff meander bends, which was found to be lower than present day discharges and then related with changing monsoonal regimes during the Holocene, as discerned from various climate proxies. Meander migration started at around 1.9 Ka, and the most recent transition to the current low-sinuous course of this river occurred at around 300 years before present, with lateral migration rates ranging between 2.4 – 4.5 m/yr. While the lower terrace surface still experiences frequent inundation impacted by higher monsoonal discharges and can be considered to be of Recent origin, deposition and abandonment of the middle terrace surfaces had occurred around the Mid-Holocene Era, indicating high incision rates by the river. This timeline corresponds to previously reported alternate strengthening and weakening phases of the Indian Summer Monsson (ISM) during the entire Holocene Period, which had caused higher monsoonal precipitation and enhanced river discharge. The sequential phases of multiple meander migration in the region also corresponds to the variability of ISM during this period. The discerned chronological sequence of the Subarnarekha River’s evolution thus suggests a close coupling between regional precipitation/climatic patterns and its induced hydrological regime, thereby highlighting the importance of palaeoclimatic studies in ascertaining river behavior.