Rivers and streams are bodies of flowing surface water that transport sediment from continental highlands to lakes, alluvial fans, and ultimately the ocean. Streams are the main agent of erosion of the earth's continental crust, and they play a major role in shaping the landscape. Streams are also a focus of humans' interaction with our environment. Human agriculture, industry, and essential biology require fresh, accessible water. Ancient human civilizations first arose in the fertile valleys

The Nile River as viewed from the space shuttle. The Nile is the longest river in the world. AP/Wide World. Reproduced by permission.

of some of the world's greatest rivers: the Yangtze and Yellow Rivers in China, the Tigris and Euphrates Rivers in the Middle East, and the Nile in Egypt. The distribution of the earth's river systems has influenced human population patterns, commerce, and conquest since then, and the availability of...