Why are many lakes in the East African Rift System deep and narrow?

Many lakes in the East African Rift System are deep and narrow because they occupy rift valleys formed by the movement of tectonic plates.

These lakes are created when blocks of the Earth's crust drop down between diverging lithospheric plates, forming long, narrow, and deep depressions known as grabens. Because these valleys are bounded by steep, fault-formed escarpments that often rise sheer from the water, the resulting lake basins are typically fjordlike and elongated in shape.123

The depth of these lakes is a direct consequence of this tectonic subsidence; for example, Lake Tanganyika reaches depths of over 4,700 feet, with its floor extending well below sea level. While some rift lakes are shallow, this is generally due to the accumulation of thick layers of sediment over time rather than the original structure of the basin.124