Vol.:(0123456789) Natural Hazards https://doi.org/10.1007/s11069-020-04156-9 1 3 ORIGINAL PAPER Characterizing cross‑sectional morphology and channel inefciency of lower Bhagirathi River, India, in post‑Farakka barrage condition Aznarul Islam 1  · Sanat Kumar Guchhait 2 Received: 21 June 2019 / Accepted: 2 July 2020 © Springer Nature B.V. 2020 Abstract The present research based on 39 cross sections, historical maps, satellite images, and intensive feld investigations intends to portray the changes in the cross-sectional mor- phology and channel efciency of the lower Bhagirathi River (Katwa to Nabadwip) after the Farakka barrage was constructed over the Ganga River in 1975. The study reveals that majority of the cross sections have undergone signifcant changes in terms of higher width/depth ratio (1972:71.4 and 1984:85.1), higher channel form index (1972:28.06, 1984:33.44), and higher areal asymmetry (1972:0.244, 1984:0.287). Thus, discriminant analysis based on the principal component analysis Z-score of the derived indices shows a reversal of the quadrant refecting a defnite change in cross-sectional morphology in the post-Farakka barrage condition. Though the Reynolds number and specifc stream power increased due to the increase in the lean period (January to May) discharges by the con- trolled hydrology, channel efciency has decreased which is refected through the higher Chezy roughness factor (1972:0.095 and 1984: 6.802) and increasing braiding index (1994:0.103, 2019:0.294). This is due to the emergence of the transport-limited condition triggered by the inadequate discharge compared to the huge sediment load contributed by Bhagirathi bank erosion and the Ajay–Mayurakshi system. Moreover, the role of erosion permitting sand and silt facies triggering higher channel oscillation along the left bank and presence of plug bar deposits coupled with the suspected attraction of subsurface fault are also noted on the relative stability and channel orientation of the middle part of the study reach. Keywords Channel form ratio · Channel asymmetry · Channel efciency · SL index · Farakka barrage · Ajay–Mayurakshi system * Aznarul Islam aznarulislam@gmail.com 1 Department of Geography, Aliah University, 17 Gorachand Road, Kolkata 700 014, India 2 Department of Geography, The University of Burdwan, Barddhaman 713 104, India