River channel changes of the Subansiri in Assam, India

Abstract

The sequential changes in the position of banklines as a result of bank erosion, as well as various changes in the channel have been studied in the Subansiri river in Assam, India. The study was carried out from available topographic maps of 1920, 1970 and satellite imagery of 1990. The types of changes taking place in the Subansiri river channel as observed from the above-mentioned maps and imagery are grouped into four categories: (1) alteration of the direction of flow due to neck cut-off, (2) widening of a channel in response to bar development, (3) development (and subsequent abandonment) of anabranches, and (4) progressive shifting of meander bends. The river channel has become substantially wider during the period of 1970–1990. There has also been a progressive increase in the number and area of mid-channel bars with a corresponding decrease in the number and area of point bars during 1920–1990. These periodic and progressive changes are responsible for a change in channel pattern of the river. A study made on nature and amount of change in the banklines of the Subansiri river by dividing the channel (84 km in 1970) into 10 equal segments reveals that the types of change are varied, and in some cases chaotic in nature. Study of the bankline shift of the Subansiri determined separately for the periods 1920–1970, 1970–1990 and 1920–1990 at 20 evenly spaced transverse sections reveals that there has been a substantial westward shift of both the banklines, up to a maximum of 6 km, in the three upstream sections during 1920–1970. However, shifting of banklines in both directions and the unequal amount of shifting in the same direction have caused a marked increase in channel width during 1970–1990. It is found that due to bankline migration, the total amount of area eroded along the entire course under study during 1920–1970 and 1970–1990 are 107.90 and 57.50 km 2, respectively. The overall sinuosity which was 1.51 in 1920 has decreased to 1.30 in 1970, while the corresponding braiding intensity has increased from 0.79 to 1.32. The river channel seems to make a remarkable change from a meandering pattern in 1920 towards a braided pattern by 1990 as seen from gradual increase in overall braiding intensity.