Obscure spate ditch in Kielder Forest. The Seven Linns section is a series of falls and slides. Other notable features are Cat Linns and Jerry's Linn (a 4m fall).
You need to get the OS map out and get familiar with forestry tracks! The Seven Linns, which are a few hundred metres long are the main event and it's debatable as to whether it's worth paddling the rest of the beck, or just going to Jerry's Linn for a one-hit wonder.
Rainchasers is best experienced as an app.