Why did many Sumerian city states develop near the mouth of the Tigris and Euphrates rivers?

Many Sumerian city-states developed near the mouth of the Tigris and Euphrates rivers due to the fertile alluvial soil that resulted from periodic flooding, which provided ideal conditions for agriculture. The rivers also offered a reliable water source for irrigation, crucial for sustaining crops in the arid climate of Mesopotamia. Additionally, their proximity to the rivers facilitated trade and transportation, allowing city-states to connect with each other and other regions. This combination of agricultural abundance and trade opportunities contributed to the growth and prosperity of Sumerian civilization.