The Early Mesopotamian's faced a lot of challenging preparing the environment for suitable living. At first the settlers in the Zagros Mountains lived in a mild climate and had plenty of wood and stone for building and tool making. Over time these good conditions allowed the population to rise to unsustainable levels. Food shortages struck the hills and forced the Mesopotamian living in the foothills to move south on to the plains of Sumer. Harsh climate, uncontrolled water supply, building maintaining irrigation systems  and attacking neighbors made teamwork with other villages vitally important for survival. Floods from the Tigris and Euphrates rivers annually wiped out crops that were planted to early leaving the Sumerians with decimated farms. The creation of irrigation systems solved many problems facing Sumerians living in ancient Sumer. Levees, canals, dams, and reservoirs made farming manageable in the region. Once dependent on hunting and gathering for food the Sumerians were able to settle on the plains and build farms that lasted generations. Over time, the irrigation systems would get clogged with silt disrupting the entire system. Farmers had to be very mindful of clogged irrigation systems, because the entire region depended on all Sumerians working together. Maintaining the water systems across village boundaries brought people together to form villages and over time the first City-States.  These City-States were like small independent countries that had its own ruler and laws. City-States often fought over the right to use more water which led to the creation of even bigger City-States with armies and city defenses.