- Runoff and Drainage

Rivers, lakes, and reservoirs created by dams provide more than 75% of the water used in the United States. Surface water systems are also used for recreational purposes and in many cases are important transportation conduits. The importance of surface water in the development of the United States is illustrated by looking at a map of the nation. Virtually all cities with populations exceeding 150,000 are located on rivers or lakes, and so are many smaller communities. There are about 2 million streams and rivers in the United States, including the mighty Mississippi River, which is the fourth longest river in the world at 3710 mi. long. Of the world’s 11 freshwater lakes with the largest surface area, four are located in the Laurentian Great Lakes system (Superior is 1st, Huron is 5th, Michigan is 6th, and Erie is 11th). In addition to the Great Lakes, we have many thousands of smaller lakes, dams, reservoirs, and ponds. These include 25 lakes with surface areas greater than 100 mi2. and more than 200 with surface areas larger than 10 mi2. All of these water bodies depend on the phenomenon of runoff.