I always associated the dipper (Cinclus cinclus) with fast running streams in the granite country of Dartmoor, Wales, northern England and Scotland so when I moced to Dorset I was surprised to find that dipper occur on the species list of some nature reserves in Dorset. Further research reveals that they are pretty rare in Dorset but do occur in the far west of the county on the upper reaches of the River Frome and on a couple of its tributaries. It is listed as occurring at Kingcombe and Brackets Copse which both have fast flowing streams running through them but how recent or reliable these records are I do not know.
The best source of information about this sort of thing is, naturally I suppose, the excellent annual report published by the Dorset Bird Club which seems to suggest a couple of pairs nest on the Frome near Frampton and Maiden Newton where the river is much wider than at KIngcombe or Brackets but still quite fast flowing. I have had a couple of forays across to the far side of the county in the hope of seeing them but, to use the 'twitchers' term for failing to spot ones prey, I dipped out!
The photograph I have I took in Wales and although it is from far the best you will see it does show the view you are likely to get before it flies off in search of food, they are very active birds.