 Where a named river is formed by the confluence of two differently named rivers, these are labelled as (Ls) and (Rs) for the left and right forks.A prime example is the formation of the River Taff from the Taf Fawr and the Taf Fechan at Cefn Coed-y-cymmer.The list is essentially a list of the main rivers of Wales (as defined by the Environment Agency) and which more or less includes every watercourse named on Ordnance Survey mapping.