wouldn't river based civilizations use ranged weapons? Sure you would still need infantrymen, but as the village or city has a natural defense in the river, i would think ranged weaponry would be better able to defend the city, until the attacking army breaks through the defenses.
But then again, this would be totally different if the river tribe was the attacking army. Just thoughts, nothing more. And you draw really well, whenever you update this, i am always coming back. Keep it up!