Take a look at a map of the United States or of any country in the world. Notice how towns and cities tend to be located next to rivers? This is no coincidence. Rivers provide water for the essential needs of both tiny towns and huge cities. There's a good chance that you live near one of our nation's large rivers. Ever wonder what rivers are the longest? Look at the graphic below to see our short list of long rivers.

(It's not so easy to define how long a river is. If a number of tributaries merge to form a larger river, how would you define where the river actually begins? Here is how we are defining river length:

River lengths or river-length data are affected not only by some of the natural and artificial causes noted in the preceding paragraph, but also by the precision of various techniques of measurement, by the scale of available maps or aerial photographs, and by somewhat arbitrary decisions. For example, the length may be considered to be the distance from the mouth to the most distant headwater source (irrespective of stream name) or from the mouth to the headwaters of the stream commonly identified as the source stream. The names of some rivers, such as the Mississippi River and the Rio Grande, are unchanged from source to mouth. In contrast, the name of the source of the Mobile River—Tickanetley Creek—changes five times before becoming Mobile River 45 miles north of Mobile Bay.