The People of the River by Edgar Wallace

NATIVE men loved Sanders of the River well enough to die for him. Some hated him well enough to kill him. These things have a trick of balancing themselves, and the story of Tambeli the Strong, and a member of that sinister organisation, the Silent Ones of Nigeria, offers an object lesson on this point.

From the Big River which empties itself into the Atlantic Ocean somewhere between Dacca and Banana Point -- this is a fairly vague location -- run a number of smaller rivers, east, west, and northerly, more or less.