How do Ethiopian monsoon rains cause the Nile flood?

The annual flooding of the Nile River is primarily driven by heavy summer monsoon rains that fall over the Ethiopian Plateau.12

These rains cause a rapid surge in the volume of the river's major Ethiopian tributaries, most notably the Blue Nile and the Atbara River. Because these rivers descend from high elevations and flow through deep ravines, the runoff reaches the main stream quickly, leading to a pronounced flood season that typically peaks between July and October.12

Historically, this surge from the Ethiopian highlands contributed the vast majority of the water that caused the Nile to swell in Egypt. While the White Nile provides a more constant, year-round supply of water, the dramatic seasonal variations in the river's level are a direct consequence of the Ethiopian summer monsoon.12