he Geographical Setting With in the great bend of the Niger the Volta river system drains an extensive area, covering most of the republics of Upper Volta and Ghana, as well as parts of Togo and the Ivory Coast. The vegetation of the Volta Basin changes gradually, from north to south, with the increase of rainfall and the lengthening of the wet season. In the upper reaches of the Volta Basin the Sahel gives way to the Sudan savannah, and a predominantly nomad population is replaced by agricultural communities. Cereal crops, mainly millet, form the staple food, and as the area is relatively free of the tsetse fly, cattle breeding is still important. The Upper Volta Basin, dominated by the two Mossi states of Yatenga and Wagadugu, is densely popu lated, reaching about 130 inhabitants per square mile.1 The density of this area, compared with that of the drier country to the north, goes some way to explain the important historical fact that the Mossi states blocked the expansion of the northern Sudanese empire.