Cataract

A shallow length of the Nile River, between Khartoum and Aswan
Real World Example
The cataracts of the Nile River were sections where the water was shallow and flowed quickly over rocks, making navigation difficult. In ancient times, these natural barriers served as protective borders for Egypt, preventing enemy ships from easily invading. They also marked the boundaries between different cultures and regions, such as Nubia and Egypt, fostering trade and cultural exchange. Today, the concept of natural barriers still matters as we appreciate and protect natural landscapes that define regions, like mountain ranges or rivers that serve as boundaries. For example, the Mississippi River in the United States functions similarly by providing natural borders between states and supporting commerce and transportation, much like the Nile did in ancient times.
Practice Version
