Does anyone know the convention that determines which point of a rivers catchment is defined as its source?
I've been looking at maps over the years and often wondered why one water course is chosen as the birth place of a river over another equal or apparrently superior tributary.
Is the decision purely based on the volume of water which is contributed to the flow or is there something more arcane going on?

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Musha rig um du rum da

Is it not the highest point at which it begins? The source of the Taff is the stream running through and from Pen y Fan in the Brecon Beacons!

The Welsh Dee is an odd one in this respect as the Lliw tributary seems the superior river when it joins the little Dee at Llanuwchllyn.


I remember finding the answer when comparing the sources of the Severn and Wye; both these rivers have their official source marked on the slopes of Plynlimon. Google, or the Ordnance Survey, will know.

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Save the softmouth trout

Thank you chaps, just checked, it's the furthest perennially flowing point of a catcment from the mouth of the river according to Wiki

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Musha rig um du rum da

The convention on river rites,