Two versions, two riverbanks
This story is told on both sides of the modern Ghana and Côte d'Ivoire border, and it is told differently on each side. The version followed here is the Asante one as summarised in modern reference works; the Agni (Anyi) version from Côte d'Ivoire reverses which brother is favoured and which is tricked. Neither is the correct version; each explains the land from the point of view of the people telling it.
Nyame's plan
Nyame, the sky god, had two sons, Bia and Tano, and when they were grown he decided to divide the land between them. He meant the rich, well-watered country to go to Bia, the son who pleased him most, and the poorer, drier land to Tano. He told his servant, the goat, to send the sons to him in the morning so that he could make the gift.
The goat's advice
The goat favoured Tano. Going to him first, the goat told him what Nyame intended and advised him to go to his father early, before his brother, in Bia's place. Nyame took the son before him to be Bia and gave him the fertile land. When Bia came later and asked for his share, the gift could not be taken back, and he received what was left: the country to the west.
The rivers
So Tano became the great river of the fertile Bono and Asante country, honoured as the foremost of the abosom, and Bia the river that flows westward into what is now Côte d'Ivoire. In the Agni telling the goat's help goes the other way, Bia gains the better share, and Tano has to take his inheritance back by force, driving his brother west. Either way, the story makes the two rivers brothers and their peoples rivals, and it closes with the division standing: what the sky god has given, he does not take back.