I have to admit that I don't really understand 'Ouse' and suspect that I am completely missing an insight into the answer.
The River Ouse
runs through Yorkshire whereas the River Great Ouse
runs in central & eastern England and I don't know if it enters Yorkshire at all along it's course.
There is a River Dove
in Yorkshire, but I don't see that as an any better answer than Ouse (in fact, a little worse, if anything), except that it fits Linton's given letters.
Please would someone explain it further?