Article: The boundaries of Somerset are largely unaltered from medieval times. The River Avon formed much of the border with Gloucestershire, except that the hundred of Bath Forum, which straddles the Avon, formed part of Somerset. Bristol began as a town on the Gloucestershire side of the Avon, however as it grew it extended across the river into Somerset. In 1373 Edward III proclaimed "that the town of Bristol with its suburbs and precincts shall henceforth be separate from the counties of Gloucester and Somerset... and that it should be a county by itself".

Now answer this question: What river forms the border of Somerset
The River Avon formed much of the border with Gloucestershire, except that the hundred of Bath Forum