The length of a river is very hard to calculate. It depends on the identification of the source, the identification of the mouth, and the scale of measurement of the river length between source and mouth. As a result, the length measurements of many rivers are only approximations. In particular, there has long been disagreement as to whether the Amazon or the Nile is the world's longest river.
The source of a river is not hard to determine because a river typically has many tributaries. Among the many sources, the one that is farthest away from the mouth is considered as the source of the river, thus giving a maximal river length. In practice, the tributary with the farthest source is not always the one given the name of the river. For example, the farthest source of the Mississippi River system is the source of the Jefferson River, a tributary of the Missouri River which in turn is a tributary of the Mississippi. However, a different (and shorter) tributary is identified as the Mississippi. When the river is measured from mouth to farthest source, it is called the Mississippi-Missouri-Jefferson. Also, it is hard to state exactly where a river begins as very often rivers are formed by seasonal streams, swamps, or changing lakes. In this article, length means the length of the longest continuous river channel in a given river system, regardless of name.
The mouth of a river is hard to determine in cases where the river has a large estuary that gradually widens and opens into the ocean; examples are the River Plate and the Saint Lawrence River. Some rivers like the Okavango or Colorado do not have a mouth; instead they dwindle to very low water volume and eventually evaporate, or sink into an aquifer, or get diverted for agriculture. The exact point where these rivers end will vary seasonally.
The length of a river between source and mouth may be hard to determine due to issues of map scale. Small scale maps (those showing large areas) tend to generalize, or "smooth" lines more than large scale maps (those showing small areas). According to the generally accepted ideal, length measurements should be based on maps that are of a large enough scale to show the width of the river, and the path measured is the path a small boat would take down the middle of the river.
Even when detailed maps are available, the length measurement is not always clear. A river may have multiple channels, or anabranchs. The length may depend on whether the center or the edge of the river is measured. It may not be clear how to measure the length through a lake. Seasonal and annual changes may alter both rivers and lakes. Other factors that can change the length of a river include cycles of erosion and flooding, dams, levees, and channelization. In addition, the length of meandering can change significantly over time due to natural or artificial cutoffs, when a new channel cuts across a narrow strip of land, bypassing a large river bend. For example, due to 18 cutoffs created between 1766 and 1885 the length of the Mississippi River from Cairo, Illinois, to New Orleans, Louisiana, was reduced by 218 miles (351 km).
These points make it difficult, if not impossible, to get an accurate measurement of the length of a river. The varying accuracy and precision also makes it difficult to make length comparisons between different rivers without a degree of uncertainty.