<P> Egypt also created irrigation systems from its local river, the Nile River, more intricate than previous systems . The Egyptians would rotate legumes with cereal which would stop salt buildup from the fresh water and enhance the fertility of their fields . The Nile River also allowed easier travel among the civilization, eventually resulting in the creation of two kingdoms in the north and south areas of the river until both were unified into one society by 3000 BCE . </P> <P> Much of the history of the Indus valley civilization, discovered in the 1920s, is unknown . Discovered in the 1920s, Harappan society remains a mystery because the Harappan system of writing has not yet been deciphered . It was larger than either Egypt or Mesopotamia . Historians have found no evidence of violence or a ruling class; there are no distinctive burial sites and there is not a lot of evidence to suggest a formal military . However, historians believe that the lack of knowledge about the ruling class and the military is mainly due to the inability to read Harappan writing . </P> <P> The Yellow River (Huang He) area became settled around 4000 BC . Many tribes settled along the river, sixth longest in the world, which was distinguished by its heavy load of yellow silt and its periodic devastating floods . Recorded Chinese history begins with the first great dynasty, the Xia Dynasty, which ruled the Yellow River valley from 2100 to 1600 BC . A major impetus for the tribes to unite into a single kingdom was the desire to find a solution to the frequent deadly floods . The Xia dug canals to channel excess water away from the area, resulting in a strong civilization with bountiful harvests and powerful leaders . The Yellow River is often called "The Cradle of Chinese Civilization". </P>

List the four river valley civilizations and the rivers associated with them