What are the Nile cataracts?

The Nile cataracts are a series of shallow, rock-strewn rapids located along the Nile River north of Khartoum. These features are caused by outcropping crystalline rocks that cross the river's course, creating obstacles that prevent the river from being completely navigable.1

There are six famous cataracts in total:

• First Cataract: Located immediately below the Aswan High Dam in Egypt; it was once an area of rapids that partially obstructed the river's flow.1
• Second Cataract: Now largely inundated by Lake Nasser.1
• Third Cataract: Situated between Dongola and the reaches leading toward Lake Nasser.1
• Fourth Cataract: Located in the middle of an S-bend in the river between Abū Ḥamad and Al-Dabbah.1
• Fifth Cataract: Part of the series of rapids found along the stretch of the river north of Khartoum.1
• Sixth Cataract: The highest of the cataracts, located at Sablūkah, where the river cuts through hills for eight miles.1

Because of these formations, the Nile is not navigable in a single continuous stretch, though sections between the cataracts remain accessible to sailing vessels and river steamers.12