Easy question, right? You’d expect a straightforward answer. But there isn’t.
One who wants to get to the source of a river will instinctively look for the spot that is furthest away from the sea. On a map his finger starts at the mouth and traces the river upstream. He ignores smaller tributaries, at every junction he follows the longest branch. The head of the final branch where he ends up this way, that is his river source.
But some take a different view. Take as an example the dispute on the source of the Mekong that has been going on for the past 15 years. It centers on the two branches of the river that are furthest upstream. Some concerned think that not only their length should be considered to decide which is the source of the Mekong. They argue that their water discharge and the surface area of their basin, among others, should also be taken into account to determine the Mekong’s ‘true’ source. Someone has pointed out that the longest of the two branches meanders a lot, reason why in fact the shortest should maybe be taken more seriously: if it discharges more water more quickly its more forceful stream will prevent it from meandering. The argument has almost been turned upside down: in fact because the one branch is the longest (by way of its meandering) it should nót be assigned as the source.
The outcome of this discussion will bring us what is called the ‘scientific’ source of the river.
Most countries have a geographical or scientific institute that deals with these matters. It is up to that organisation to proclaim A source to be The source. China has two of these institutes.
All of this quite apart from the beliefs of local people. They have been living in a source area for centuries and as long as can be remembered they regard (often: venerate, or even worship) a specific pool, well or beginning stream as the source of the river. The scientists’ criteria are irrelevant to them.
Scientists call this the ‘spiritual’ source.
So whoever wants to get to a river’s source can choose whose lead to follow. The rationally disposed can go along with the scientists, those that feel bureaucrats matter can adhere to the decisions of officialdom, the spiritually inclined can stick with the indigeneous population of the source area.