The feature that forms when a river overflows and submerges low-lying land on both sides is called a floodplain.

Here's a breakdown:

* Floodplain: A flat area of land adjacent to a river that is subject to periodic flooding.

* Formation: When a river overflows its banks, it deposits sediment (like silt, clay, and sand) on the surrounding land. Over time, these deposits build up, creating a relatively flat area known as the floodplain.

* Importance: Floodplains are incredibly important for ecosystems and human society:

* Biodiversity: They support diverse plant and animal life due to the rich, fertile soils.

* Agriculture: Floodplains are often used for agriculture because of the nutrient-rich soil.

* Water Storage: They act as natural reservoirs, absorbing and storing excess water from floods, which helps reduce flood damage downstream.

Let me know if you have any other questions about rivers and their features!