12.3.4. Lateral Migration of Alluvial Rivers Some alluvial rivers have shown a tendency for lateral migration over a period of years. Some rivers, which might have had very little lateral movement over a long period of time, may suddenly start migrating laterally during a succeeding period. Such changes in river courses have been found in the Yellow river, and in other rivers of China (5). Lateral movement of the Kosi river in India offers a classic example of the lateral migration of alluvial rivers (3). The Kosi river brings in a large amount of sediment load which it is unable to transport. In the process of building up an inland delta in the plains, it shifted over 110 km westward during the period from 1736 to 1964 (3). However, it did not shift from its original position at the Belka hills (near Chatra where the river leaves the Himalayas and enters the Gangetic plains) and also at Kursela (close to the confluence of the Kosi and the Ganga rivers). The lateral migration of the Kosi river was arrested by the construction of levees. Table 12.2 gives the average rates of lateral migration of the Kosi river for the period 1736-1950.
Observations of the migration of the Kosi river and the Yellow river indicate that the lateral migration of alluvial rivers is mainly due to the excessive sediment load during the floods, large variation in the river discharge, large slope, and geologically young rock formations
(5).
