If your fishing a river in spate, you can go heavier, as the additional water and colour will help hide your line. Perhaps you would also like to be prepared for the occasional salmon you may hook while fishing for sea trout. If the water is high and dirty, 10 to 15 pounds as Morphfly suggested above sounds bang on.
When the river becomes lower and clearer, my experience is that it’s only worth fishing at late in the evening/night, as it’s a small river, you will be very close to the fish, and as the water is clear, they will see you very easily. They have very good eye sight and lay in parts of the river where they can better take care of their selves.
I would only go down to 6 pounds, if I only expected to catch small sea trout, and the water is clear, using small flies in daylight. I wouldn’t expect to catch much under such conditions.