Rivers and streams are complex. They are filled with a dizzying array of life, from relatively simple bacteria and algae to more complex forms such as plants, insects, and fish. When everything is working well, rivers provide many benefits to society, including fishing and other recreation opportunities, drinking water, irrigation, and transportation. However, there are also many ways that people can disrupt the natural functioning of rivers. For example, it may be necessary for cities to build systems that rapidly remove rain water to prevent flooding damage to homes and businesses.