The Peruvian littoral appears an "improbable, even aberrant" candidate for the "pristine" development of civilization, compared to other world centers. It is extremely arid, bounded by two rain shadows (caused by the Andes to the east, and the Pacific trade winds to the west). The region is punctuated by more than 50 rivers that carry Andean snowmelt. The development of widespread irrigation from these water sources is seen as decisive in the emergence of Norte Chico; since all of the monumental architecture at various sites has been found close to irrigation channels.