They dug holes to move it and they built dams to make sure they had enough water but there crops didn't flood.
Irrigation
Farmers take or divert water from a river for irrigation purposes, and to water their crops.
Replenish the soil and provide water for the crops.
They used the rivers' water for irrigation.
Irrigate [water] their land to grow crops.
The rivers were so important to the farmers in Mesopotamia because then the farmers can get water for their crops in order to trade for goods and services.
The two rivers were the Tigris and the Euphrates Rivers.
Neither
through the system of irrigation
food and water
Irrigation
They flow into 'The Tigris' and 'Ufratis' rivers!