A More Rational Approach for Morphological Characterisation of Kosi River and Its Plains

Kosi river is a perfect example to study fluvial geomorphology. The river alone exhibits many of the fluvial morphometric features in its short journey to River Ganga. At reach scale, it creates various planforms such as braids, anabranches and meanders whereas, at the basin scale, it forms a tributary river network as well as a distributary alluvial megafan.

The present study proposes a new perspective in understanding the geomorphological elements on the Kosi River and its basin using remote sensing and hydrodynamic techniques across the various relevant scales. The research also investigates the changes in the Kosi river morphology and ascertains the causal physical phenomenon for these changes. Topographic maps, LANDSAT Imageries and SRTM Digital Elevation Model were used for the detailed examination of the river and its basin morphometry.

The analysis reveals that while Kosi has many feeding tributaries along its western flank (like Baghmati, Kamla, Balan, etc.) whereas, along its eastern flank, there are none. Thus alluding to dominant evidence of drainage asymmetry in the Kosi plains. The study also seeks to classify the river-reaches by their respective planforms. The reaches in these planform zones have different evolution dynamics and thus, also have significant differences with regards to aspects related to channel stability. The study highlights the inference that it is only the lower reach of the river, confined to the meandering zone, which is morphologically stable. On the other hand, the upper and middle reaches are rapidly evolving and seemingly in a state of perpetual transition and therefore deemed to be in a critical state.

The results also show that the construction of barrage has caused a significant effect on the upstream reach where hydrodynamic conditions tend to promote the natural process of sediment deposition which, in turn, has resulted in nudging the river ever so close to its eastern embankment and accordingly an eastward shift of the river by as much as 4 km. In its middle reaches, the river develops anabranching patterns and is highly dynamic. In this zone, the river has shown gradual westward movement and is skimming along its embankments. The study also reveals that Kosi and Baghmati do not have a definite point of mutual confluence but form a continuously evolving looped network structure.