Why did rivers in southern Mesopotamia often flood or change course?

In southern Mesopotamia, the Tigris and Euphrates rivers frequently flooded and shifted their courses due to the unique geography of the low-lying alluvial plain.12

Because the terrain between Baghdad and the Shatt al-Arab has a very slight elevation gradient—dropping only about 100 feet over a vast distance—the rivers flow slowly. This sluggish movement causes heavy deposits of silt to accumulate, which gradually raises the riverbeds above the level of the surrounding plain. Without the protection of high dikes, these elevated rivers easily overflow their banks.12

Furthermore, the rivers are subject to significant seasonal volume changes driven by winter rains and spring snowmelt in the northern mountains. The Tigris, in particular, is known for being swifter and more silt-laden than the Euphrates, making it highly unpredictable and prone to destructive flooding. Over millennia, these natural processes, combined with human irrigation interventions, have caused the rivers to abandon old channels and carve new paths across the landscape.34521