DBQ helppppppppp!!!!!!!!!!!!!!!!!!…

okay so /I am doing a dbq but i suck at getting outside information. my dbq is about geographic features like the Tigris and Euphrates river i already have outside info on the Indian monsoon. uhhhhhh i need to know WHY they made such a big impact on the societies that settled around them. please help i know there in the fertile crescent and that they make fertile soil, is there anything else i cant put in my dbq to make it better plus i need the outside info its part of my grade!!?