 the first tributary listed is closest to the sea, and tributaries of tributaries are treated similarly.Thus, in the first catchment below, the River Sark is the lowermost tributary of the Border Esk and the Hether Burn is the lowermost tributary of the River Lyne.The main stem (or principal) river of a catchment is labelled as (MS), left-bank tributaries are indicated by (L), right-bank tributaries by (R).