Answer: Unpredictable flooding
Explanation: The early civilizations grew out of the fact that they were located in the river valleys, which was a prerequisite for the development of agriculture, consequently, and the development of the civilizations themselves. This meant that the rivers in the valleys occasionally poured and made the soil fertile, that is, suitable for agriculture. However, there were unforeseen floods that were not common at certain times of the year when they would destroy yields and even settlements. These were problems that early civilizations faced, although they resorted to the construction of canals and irrigation systems to mitigate as far as possible the catastrophic consequences of unforeseen floods.