The short answer is by using boats with a shallow draft ( and therefore not able to carry large amounts of weight) and by accepting that for weeks, maybe months at a time transport would be halted by low water levels. Further downstream from Staffordshire the Trent has more flow and was able to support more and larger freight traffic - as an example from the 15C at least coal was transported from the Wollaton area to Nottingham via the River Leen and Trent. If you follow the course of the Trent you will find many places where navigation was improved in the 18C by building weirs to raise water levels over problematic sections. An excellent example but rather to the east of Staffordshire is at Sawley where a weir raises the water level above a shallow section where there was a road crossing from the Roman period ( and probably before). Oh, and just remember a Viking fleet was able to navigate the Trent as far as Repton in 873.