A flooding river brings a lot of rich mud, which is deposited in the agricultural fields of a people group.  This means that the fields will grow more and better crops, something absolutely necessary for an fledgeling civilization.  Complex civilization is defined by one thing primarily: craft specialization.  This is when a society has plenty of food, and some people can devote that time and energy into other crafts, such as art, weaving, metalworking, etc.  Hunter-gatherer societies simply do not have time for people to specialize in a trade, as each person must obtain their own food.  When a society is capable of having food-production specialists who can produce a surplus, the door is open for other specialists to invent and develop other trades, because they can simply buy food obtained by other people.
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