Peter Turchin explains the forces behind imperiogenesis, the formation of empires:
Take the case of the Sinic (Chinese) civilization. Over the last three thousand years the cradle of the Chinese civilization, the Yellow River Basin, has been unified by one empire after another. There is no other region on Earth that could rival the Yellow River Basin in the intensity of ‘imperiogenesis’ (proportion of time that it found itself within a large empire). In a series of publications (for example, this one) I have argued that the explanation of this remarkable pattern has to do with the very intensive warfare between nomadic pastoralists (Hunnu, Turks, Mongols, etc) and the agrarian Chinese. This is why China was typically unified from the North (and most frequently from the Northwest) – it was the military pressure from the Great Eurasian Steppe that selected for unusually cohesive North Chinese societies, which then would go on to build huge empires by conquering the rest of East Asia.
Steppe frontiers are crucibles of empires; you add a major river and you are practically guaranteed to have an imperiogenesis hotspot. Examples are numerous: the Nile, the Tigris-Euphrates, and the Indus are the usual suspects. But the first empires in sub-Saharan Africa (Ghana, Mali, and Songhai) arose on the Niger River where it flows through the Sahel. This correlation has been long noted. Karl Wittfogel attempted to explain this observation with his theory of ‘hydraulic empires’, based on the control of irrigation by state bureaucracy, but this theory has been empirically disproved. For example, the major river of eastern Europe, the Volga, was the cradle of a number of empires (Bulghar, the Kazan Khanate, and, most notably, Muscovy-Russia), none of which relied on intensive irrigation. Nor did the Chinese along the Yellow River. In other civilizations irrigation was typically a local, rather than an imperial concern. Most likely, the river effect is due to a combination of good environment for intensive agriculture on alluvial soils and the ease of communications (because transporting goods on water was an order of magnitude cheaper than carting them on land).