Additionally, it is not uncommon for monarchies to contain democratic institutions at the local level. For ancient Near Eastern examples, see Andrea Seri’s Local Power in Old Babylonian Mesopotamia and Daniel Fleming’s Democracy's Ancient Ancestors: Mari and Early Collective Governance.

In any case, a geographic argument is rather dubious. It should be noted that early states in Greece like the Mycenaean kingdoms were in fact monarchies, and kingdoms developed in many regions without unifying rivers — the Canaanite city-states of the Levant, the kingdoms of ancient Cyprus like Idalion and Paphos, Elam and the other kingdoms of ancient Iran, the Anatolian city-states of the Early/Middle Bronze Age and the subsequent Hittite empire, etc., to say nothing of societies further afield like the Maya kingdoms of Mesoamerica.