The author's argument in The River Between is generally about the divide between the tribal culture in Africa and the Christian culture (and also about how colonialism has deepened that divide).
Many people believe that colonialism, or the conquest of Africa in order mostly to "Christianize" the nation, actually ruined the continent. Traditional ways could not be married with modern ones, so the result was basic destruction.
When the white man came and fixed himself in Siriana, I warned all the people. But they laughed at me. Maybe I was hasty.
Now the younger members of the population don't appreciate the old and traditional ways. And Waiyaki makes the final realization that a "white" education can't achieve unity among the tribes and that politics is the only way to preserve the culture. However, the author, Ngugi does not suggest lingering over sentiment of the lost past.
Generally, Ngugi is trying to mold the current image of Africa that has been marred (Ngugi believes) by writers such as Joseph Conrad and others who portray the old ways as evil.