Anuak traditions suggest that they migrated into their present country from the northwest, beginning some four hundred years ago. It has been suggested by a number of authorities that the ecological variation that exists within the territory they came to settle has had a significant impact on the course of intracultural variation among different Anuak settlements. In the northwestern regions of their country, the land is low and flat and is thus subject to seasonal flooding from the Pibor and Sobat rivers, which feed the White Nile. Settlements in this part of Anuak country tend to be dispersed and isolated, particularly during the season of rains and floods (April to September). Farther to the south and east, however, the elevation of the countryside increases. Here, hamlets are typically in closer proximity, and communication between different settlements is more frequent throughout the year. The population densities of hamlets in the southeastern part of the country are notably higher than in the northwest. Villages are often surrounded by wooden enclosures.