Question

what was the benefit of the river flooding? created a calendar killed pests fertile soil washed away weeds

Explanation:

River flooding deposits nutrient - rich sediments, which make the soil fertile. Creating a calendar is not related to the direct benefit of flooding. Killing pests is not a typical positive benefit of flooding (flooding can displace pests but not necessarily kill them in a beneficial way). Washing away weeds is not a major recognized benefit compared to soil fertility.

Answer:

Fertile soil