Why do the courses of rivers in Bangladesh change over time?

The courses of rivers in Bangladesh change over time primarily due to the constant processes of erosion and the deposition of massive amounts of silt. As these active rivers carry enormous volumes of water and sediment—the highest combined suspended sediment load in the world—they frequently shift their channels, causing the landscape to evolve continuously.12

Specific factors contributing to these changes include:

• High Sediment Load: The rivers, particularly the Brahmaputra and Ganges, carry heavy loads of silt that build up temporary islands and new lands known as chars. These deposits alter the flow and force the rivers to carve new paths.12
• Monsoon Flooding: During the wet monsoon months, the rivers overflow their banks. Exceptional flooding events, such as the 1787 Tista flood, can cause sudden and dramatic diversions, forcing a river to abandon its old course and adopt a new channel.13
• Hydrological Dynamics: The speed and volume of water, reinforced by tributaries, push channels into new positions. For example, silt-laden northern tributaries can push the main channel of a river toward the opposite bank.1
• Geographic Instability: Because the region is low-lying and composed of unconsolidated sediments, the rivers are rarely in the exact same place for two successive years, making the topography dynamic rather than static.132