Why were many early Malagasy capitals located near river mouths?

During the 16th and early 17th centuries, many early Malagasy capitals were located near river mouths because these coastal areas were central to the island's international trade networks.1

These locations facilitated participation in an Afro-Arab commercial network that spanned the western Indian Ocean. By establishing settlements at river estuaries, local rulers could effectively engage in trade and interact with visiting ships, which frequently stopped at these accessible coastal points. While the island remained politically fragmented during this period, these river-mouth towns served as the administrative and commercial hubs for small, independent local states.1