Anthropogeomorphology of the Bhagirathi River

The Bhagirathi River Basin is profoundly controlled by the Farakka Barrage constructed in 1975 over the Ganga River. The Indo-Bangladesh water-sharing treaties of 1977 and 1996 resulted in a fluctuating river regime on ten-day scale in the lean period (January–May). This typical regime affected channel behaviour mainly by changes in width (1975: 230 m; 2004–2008: 278 m), cubic capacity (1975: ~15 × 10³m³; 2005–2009: ~20 × 10³m³), hydraulic mean radius (1975: 3.16 m; 2004–2008: 3.44 m) and cross-sectional area (1975: 723 m²; 2004–2009: 941 m²). Furthermore, bank erosion, channel oscillation, meander cut-off and braiding are the other imprints of the Farakka Barrage Project (FBP). In the post-Farakka period, the river Bhagirathi received a considerable amount of water in variable quantities from the river Ganga, especially in the lean period which has steadied the river bank erosion in the year round. Thus, all the four meander cut-offs occurred after 1975. The tendency of braiding has also increased especially in the lower reach of the river. Naturally, bed scouring and bank erosion at the upper and middle stretches and its eventual bed deposition in its lower part are found due to inadequate supply of discharge of about 25,000 cusecs against the recommended discharge of about 40,000 cusecs. Therefore, the bed slope has decreased in the post-Farakka period. Besides, other small-scale channel interventions in the form of urbanisation, brick field industries, road-stream crossing, guide bank and ship movements have altered the channel. Urbanisation near Berhampore, Katwa and Nabadwip has impeded oscillation of the river compared to its upper or lower parts. Brick fields have typical sediment-trapping depressions modifying flow dynamics and sediment flux. Road-stream crossings are found to alter channel bed morphology by inducing alternating erosion and accretion. Guide bank near Nabadwip has induced anthropogenic bar formation above the structure. Moreover, due to the ship movements in the national waterway number 1 the collapse of the river bank has become recurrent.