How do waterfalls and rapids prevent fish from moving between some East African lakes?

Waterfalls and rapids act as physical barriers that prevent the movement of fish between certain East African lakes, effectively isolating different faunal provinces.

For example, Murchison Falls serves as a physical barrier near the northern end of Lake Albert, marking the boundary of a distinct faunal province that separates it from Lake Victoria and Lake Kyoga. Consequently, several fish genera found in Lake Albert are absent from these two lakes, which instead host many unique species.1

Similarly, the rapids on the Semliki River have prevented fish species from migrating from Lake Albert into Lake Edward and Lake George. In addition to affecting fish distribution, such falls in outflow rivers have also sheltered certain lakes, such as Lakes Edward, George, and Lake Kivu, from the spread of crocodiles.1