In common with the headwaters of the Biel Water it rises on the low hillside of Clints Dod (122 m or 400 ft) in the Lammermuir Hills, just ESE of Whitecastle Hillfort and 3 km (1.9 mi) south-east of the village of Garvald.

Adder may be derived from Brittonic *ador, *edir or Old English edre,[2] possible ancient hydronymic terms derived from an Indo-European formation meaning "a watercourse, a channel" (compare River Etherow).

The stream wends its way south east for approximately 5 km (3.1 mi) where it joins with the Faseny Water to form the Whiteadder Reservoir created in 1968, which supplies most of the towns of East Lothian (including, before its closure, the Cockenzie power station) and Berwickshire, with water.

By this point having become a much larger body of flow, the Whiteadder meanders across Eastern parts of the Merse passing the communities of Preston, Chirnside, Allanton.

[4] The lowermost section of the Whiteadder is a Site of Special Scientific Interest (SSSI) by virtue of its water crowfoot, salmon, lamprey and moulting mute swan.