Please answer a question about the following article about Sumer:

The Sumerians were a non-Semitic caucasoid people, and spoke a language isolate; a number of linguists believed they could detect a substrate language beneath Sumerian, because names of some of Sumer's major cities are not Sumerian, revealing influences of earlier inhabitants. However, the archaeological record shows clear uninterrupted cultural continuity from the time of the early Ubaid period (5300 – 4700 BC C-14) settlements in southern Mesopotamia. The Sumerian people who settled here farmed the lands in this region that were made fertile by silt deposited by the Tigris and the Euphrates rivers.

Why were the lands in the region the Sumerian people settled fertile?
silt deposited by the Tigris and the Euphrates rivers