Which practice is closely associated with most ancient river valley civilizations?

The practice of farming is closely associated with most ancient river valley civilizations. The rivers brought fertile soils every year and this made farming a great option. The banks of the rivers were flooded every year during the rainy season and when the water receded, the fertile soil was left behind. This soil was very rich for any kind of cultivation. This also led the people to settle beside the river banks from their nomadic nature. Now they did not have to go in search of food as they could cultivate food for their self sufficiency. This resulted in ancient civilizations getting started beside river valleys.

The answer is ethnic groups.