 The Frome originally flowed east of its present @-@ day course from Stone Bridge ( now underground by St John 's Gate ) along the line of St Stephen 's Street and Baldwin Street , joining the Avon near Bristol Bridge . The narrow strip of high land between the two rivers was a naturally strategic place for the Saxon town of <unk> , later the walled centre of the city , to develop . When Robert , 1st Earl of Gloucester , rebuilt Bristol Castle , the Frome was diverted ( at present @-@ day Broad Weir ) to form the castle moat , so that the city was entirely surrounded by water . 
