There are various modern rivers, or parts of rivers, in modern China known by a name meaning Red River. In ancient times, the sources of these rivers often were not known by people down stream, and they often created mythological explanations for the sources of these rivers, such as mythological rivers flowing from Kunlun. For example, the official source of the longest river in Asia: the Yangzi (Yangtze, in older versions) main head water source is what is sometimes known in Mongolian as Ulaan Mörön, meaning Red River (it is also known by other names, such as the Tuotuo River). It is worth noting that the use of Yanngzi to refer to the whole river is relatively recent, classically and in ancient times, this was only used to describe the lower reaches, with other names used for the upper stretches. Another of Asia's major rivers is also named the Red River (; ). With a source in the Hengduan Mountains, it flows into the ocean through northern Vietnam. The study of the relationship between real rivers and mythological rivers is complicated by the various name changes or variations in names for the real rivers over time, including often interchangeable Chinese characters for water, river, or stream. Although the relationship between real and mythological geography such as rivers is not always clear, but one open to further study.