Most of us has some degree of curiosity about where things start and end.  The Amazon River is the world's biggest river by far, in terms of the  volume of water that reaches the sea, but only recently has its source been  discovered. Rivers end when and where their mouths empty into a larger body  of water, either another river, a bay, or an ocean. Knowing the location  of a river's mouth is, generally, pretty straightforward. On the other  hand, finding the exact source of a river may require both legwork and  modern mapping methods.

In earlier times, mapping a river was especially useful for drawing up  boundaries. For example, the Potomac River has been used as the boundary  between Maryland and Virginia ever since they were first established as  colonies. The source of the Potomac River, or its headwaters, is in the  Appalachian Mountains of West Virginia (formerly Virginia), and it was  traced by surveyors more than two hundred years ago, when the headwater's  area was still wilderness.

Finding a river's origin in the most inaccessible reaches of the world has  intrigued explorers for centuries. This past year, a 22 member  international team of mappers and explorers, sponsored by the National  Geographic Society, believes that they've pinpointed the source of the  Amazon River. The team explored five different headwater streams in the  Andes before they were convinced that they had indeed discovered the place  where drops of water first collect to form the mighty Amazon. According to  the team, the Amazon's origin is a small mountain stream that flows from  the flanks of Nevado Mismi, an 18,363 foot mountain in southern Peru. A  global positioning system (GPS), linked to a network of satellites, was  employed to precisely locate the source of the Amazon, which is less than  100 miles from the Pacific Ocean.

The Amazon is approximately 6,500 miles in length, second only to the Nile  River in Africa. However, no river comes close to the Amazon in terms of  how much water it delivers to the ocean. The Amazon likely carries 60 times  more water than does the Nile. Much of the Amazon's water is provided by  the presence of the Intertropical Convergence Zone (ITCZ), a band of  convective clouds and thunderstorms nearly paralleling the Equator. For  most of the year, the ITCZ is positioned over the Amazon basin, and  rainfall is a near daily occurrence. The Amazon basin may be the only place  in the world where large amounts of precipitation occurs over the interior  of a continental lowland. So much sediment moves through the mouth of the  Amazon that the edge of the continental shelf extends outward for about 200  miles into the Atlantic Ocean.

The origin of most of the Earth's great rivers have been known for some  time. Although, Nevado Mismi has been thought to have been the source for  the Amazon since 1971, another stream was also in contention for this  honor. What has made determining the source of the Amazon so difficult has  been a combination of unfriendly terrain, high altitudes, cold winds, and  the large number of potential headwater streams that needed to be  investigated. If you've ever tried to follow a creek upstream looking for  the spring that marks its source, you've probably realized that deciding  which branch to take is no simple matter. At the confluence of two creeks,  do you take the larger one or the one that seems to be flowing faster?

What defines a river's origin is the most distant point from the mouth  (measured along the river's course and not by the way the crow flies) that  flows year-around. Of the five major Andean streams that come together to  form the headwaters of the Amazon, the Apurimac is the longest, but until  last year, it wasn't positively known which of the many branch streams that  feed the Apurimac was the most distant from the mouth of the Amazon. Other  branches of the Apurimac were slightly longer than the unnamed stream on  Nevado Mismi, but they didn't meet the criteria of flowing throughout the  year. The point of origin may or may not be the stream that has the  greatest volume of water or has the fastest flow.

The source is basically the farthest point along the main trunk of a river  - not including the length of major tributaries. For instance, the  Mississippi River is not quite as long as its major tributary, the Missouri  River. Moreover, the Missouri combined with the Mississippi at their  confluence is considerably longer than the Mississippi River itself.  Nevertheless, the source of the Mississippi River is in Minnesota (Lake  Itasca to be exact) and not in southwestern Montana, where the Missouri  begins.

For more about this see the National Geographic Society -  http://news.nationalgeographic.com/