Avulsion and shifts in river courses are characterized by scouring of riverbed and banks. These phenomena are common in case of Himalayan rivers and have been on the increase in the Anthropocene. Humans can completely alter a river system turning it into a controlled one by damming its course, by flow diversion, by dredging or clearing the channel, by straightening the channel pattern or by increasing its supply via an artificial channel. In order to control flooding and river bank erosion, channels are frequently modified. Modifications of channels often include channel widening, channel deepening, artificially cutting meanders and even channel straightening. This jeopardizes the hydrologic regime. Also, if channelization occurs, higher flows are often found downstream as the water rushes out of the channelized reach. High backwaters are resulted, resulting channel deposition lowering the channel capacity.  

A direct change in the river system is caused when dams are constructed. The hydrologic regime is altered by flow regulation, decreasing peak flows and increasing low flow. As a result, sediment transport capacity is modified inducing erosion and deposition affecting channel morphology. Dams also cause deposition in and above the reservoirs which thereby lose their capacity to retain water. Spillway water from dams is devoid of sediment load and thus tends to erode the channel immediately downstream. 

The case of Aswan dam can be cited as an example where scour erosion is noted in the lower reaches, with a lean sediment transport and downstream erosion as the flow is regulated. Increase in depth and decrease in width is also reported for other channels which have experienced dam construction. 

Morphologic discourse of the Ganga valley leads to its classification into 7 segments (Singh and Singh, 1992). Variation in discharge, sediment load, ground slope, anthropogenic activities and tectonics result in variation in channel morphology (Valdiya, 2003). Geology, geomorphology, climate alongwith human influence modifies fluvial dynamics. The outer Himalayan belt consists of recently uplifted Siwalik of Miocene Pliestocene detritus filled sediments – coarse sandstones, clays and conglomerates. Except the Siwaliks, the concerned region is a portion of the Ganga megafan and this surface shows several north-south aligned drainage channels (Shukla et al., 2001). This region is deeply incised by the active Ganga River and other rivers. Active incision of channel over the megafan surface is evidenced by the prominent channel scarps. 

Over the years, channel sedimentation with the formation of channel bars in Ganga and other rivers is observed. Decline in the number of bars accompanied with increase in size of the bars indicates rapid silt deposition. Sandbars are observed to change occasionally during floods. The shifting nature of Ganga channel is evidenced by the presence of an abandoned channel within the cross section situated below the Chandi bridge, located on the left bank of river Ganga at Nildhara area. 

The Bhimgoda barrage is located on river Ganga at Har ki Pauri. It was built as the headworks of upper Ganges canal. The initial barrage was completed by 1854. Replaced twice, the barrage was completed in 1983. The primary purpose of the barrage was irrigation; it was also used to generate hydroelectricity and used for flood control. The barrage has a length of 454 m and a spillway capacity of 19,300 m3/s. 

The local geomorphology of the Ganga River is significantly altered due to the construction of the Bhimgoda barrage as it results in huge diversion of water into the upper Ganga canal. This leads to excessive in channel sedimentation in both upstream and downstream courses interrupting the natural channel equilibrium. As a result, the flow velocity is reduced in the lean periods with the reduction in monsoonal rainfall. The braided floodplain dynamics also gets altered as a result. 

The Bhimgoda barrage diverts the the river water into the upper Ganga canal for hydroelectric power generation. Studies indicate that water velocity of river Ganga is about 1-1.9 m/s in the Haridwar region after the Bhimgoda barrage (Kumar et al., 2023). 

There have been drastic changes in water discharge, sediment concentration and calculated sediment load before and after the construction of the Bhimgoda barrage. The water discharge was uninterrupted with high velocities (reaching up to 1.5-2 m/s), with massive flushes during monsoon. Post construction, this discharge has become fractioned upto 300-400 cumecs are permanently diverted out of the river into the canal system. Sediment concentration was highly variable before the construction (varying from <50 mg/l in winter to >2000 mg/l in monsoons). Post construction sediment concentration has become high upstream of the gates, heavily diluted getting finer downstream due to sediment trapping. The calculated sediment load used to pass uniformly downstream scaling up to 8 million tonnes/day across the upper Ganga. Post construction this has been disrupted, massive trapping upstream now leads to a drastically lower calculated sediment load footprint in the downstream riverine stretch. 

Braiding index according to Brice has been calculated for this stretch for the years 1916,1972,2005 and 2026. The braiding index in 1916 was 7.347, for 1972 it was 6.76, in 2005 it was 5.4779 and in 2026 it increased to 6.26. 
                                                                         

Increase in braiding index indicates that a river is shifting from a single channel into a more complicated network of multiple branching channels which rejoin each other and are separated by islands or sandbars. Often this indicates higher sediment loads, unstable banks and changing water flows. There is also more fluctuation of discharge. Factors like flatter slope leads to a velocity drop, initiating sediment deposition and weak riverbanks which can collapse easily widening the active channel belt. Increase in stream power can also cause increasing braids.

Anthropic interference in the form of river engineering works thus often lead to modifications in fluvial dynamics interrupting the morphologic equilibrium of rivers. 

References: 

1. Dutta R.K. (2017): Controlling Factors of Channel Shifting and Avulsion in Haridwar District, Uttarakhand Geographical Review of India 79(2) June 2017, 153-167 

2. Kumar D, Kumar A, Malik D.S., Sharma R and Gupta V (2023): Effect of Barrages and Anthropogenic Activities on Ecological Integrity of the Ganga River: A Review on 

Current Issues and Restoration Efforts AgroEnvironmental Sustainability, 2023, 1(1), 67-75 https://doi.org/10.00000/s2023010109 

1. Shukla, U. K., Singh, I. B., Sharma, M., & Sharma, S. (2001). A model of alluvial megafan sedimentation: Ganga Megafan. Sedimentary Geology, 14, 243–262. 

2. Singh, M., & Singh, I. B. (1992). The Ganga River Valley: Alluvial valley in an active foreland basin. In 29th International Geological Conference, Kyoto, 2 (p. 30). Japan. 

3. Valdiya K.S. (2003): Reactivation of Himalayan Frontal Fault, Current Science, 85(7), 2001 1031-1040. 
   
   ABOUT AUTHOR
    

   

Dr Karabi Das, Masters in Geography from University of Calcutta, former Senior Research Fellow, UGC, PhD on Physical and Socioeconomic changes in the Indian Sundarban is presently working as Assistant Professor of Geography, Dr Kanailal Bhattacharyya College, Howrah.
She has participated in many national and international seminars and has 12 papers and 10 book chapters to her credit.
Her areas of interest include Fluvial Geomorphology, river in equilibrium and human environment relationship.