Why does the Atbara River shrink to pools during the dry season?

The Atbara River shrinks to a series of pools during the dry season primarily because it loses a significant portion of its water to evaporation and transpiration after passing the Khashm al-Qirbah dam.1

Unlike the Blue Nile, which is a perennial stream, the Atbara is highly seasonal. Its flow is heavily dependent on rainfall from the northern Ethiopian Plateau; during the dry season, the lack of sustained precipitation and the high rates of water loss cause the river level to drop significantly, leaving only isolated pools in the riverbed.21