Identification: the river Duddon, Cumbria
The Celtic name of the river Duddon will have been of the form Tuton, where the first river-letter t corresponds to the hill-letter l1 and the second to the hill-letter l2. But as in the case of several other rivers this river had an alternative name comprising a river-suffix attached to the name of a settlement located on the river. That alternative name is given as Coantia by Ravenna. The river-suffix is the river-letter t corresponding to the hill-letter l2. We can thus be fairly confident that the name of the settlement was Colan, where the element col means ‘steep hill’ and uses the hill-letter l1. In other words Colan was an Iron Age settlement adjacent a steep hill on or close to the river Duddon. In giving the form Coantia Ravenna has simply omitted the intervocalic l of Colantia. But it was the river-name comprising only river-letters, Tuton, which survived, with the first t changed to d (a common change) and the second to dd (as in the river Nidd).
[NB. Detailed information as to the different river-letters and how they were combined to form compound river-names, together with information as to the four categories of Celtic river-names, is given in Chapter 19: the rivers of Roman Britain. Detailed information as to the different hill-letters is given in Chapter 1, and information as to how these hill-letters were combined to form compound place-names is given in Chapter 2].