What physical feature is the primary factor determining settlement patterns in North Africa and Southwest Asia?

This physical feature would be deserts. In Northern Africa this desert is the Sahara Desert. A desert is a region that receives extremely low rainfall annually. Settlements can only be supported in these regions if there is ready access to ground water or if a river flows in the vicinity. Using the Nile in Egypt as an example, one can see that the settlement pattern is relatively linear, following the course of the river.