Since the rivers that flow to the coast almost always have their origin in the sertão, a drought in the semiarid and arid sub-regions can extend to the entire Northeastern area.[13] Prior to Portuguese settlement in the 16th century, there were no pressing problems because the predominant caatinga ecosystem was adapted to the cyclical climate. Indigenous groups who inhabited the land were affected by extreme droughts, which forced them to move to the humid coastal region, but these rarely occurred.