Average rainfall within the catchment decreases markedly from west to east. Around its source in the Tweedsmuir Hills approximately 2000 mm fall per annum whilst around the lower reaches on the Merse of Berwickshire the rainfall is approximately 650mm per annum.
In the higher, wetter and steeper catchments of the south and west of the catchment, rivers such as the Ettrick and Jed Water tend to rise and fall more quickly and have characteristic waterfalls and rapids. Eastern tributaries such as the Till, Eden and Leet, have drier catchments with gentler slopes and fewer rapids and riffle stretches.
The Scottish Environment Protection Agency operates 80 rain gauges and 26 river gauging stations in the Scottish part of the catchment, while the Environment Agency has responsibility for this across the Border.
