Why would you need to pass a law to alter the river in order to carry out works to make to make it navigable, and to levy charges?
There are two reasons
- Firstly the rivers were the property of the crown, upon which the subjects of the realm had free passage, as on a highway. To make any work which would obstruct this right, or to charges tolls, required Royal Consent, as given in an act of Parliament. This was the same when bringing in turnpikes, where charges were made for roads, and both navigations and turnpikes were for a time treated equally.
- Magna Carta had outlawed obstructions to the free passage of vessels (and later fish) in all rivers. To place a lock or weir would be an obstruction and illegal without an act authorizing it.
Certainly to raise the tolls to pay for the works, certainly to overcome the loss to landowners should the works affect their property, but one other interesting reason is quoted in 'Wisdom's law of waters (5th Edition 1992)' Obstructing a navigable watercourse by creating a lock or weir was illegal on any river because of a clause in the Magna Carta of 1225 as contested in the King V Clark (1702). The clause (33) in the charter was for the removal of all fish weirs except on the coast. A later revision only permitted those that had been present in the reign of Edward 1st. So in the 1600's and later if you wanted a new weir or lock to make the river passable for larger vessels, it would have to be legislated for to avoid breaking this ancient law.
Why did the Magna Carta call for such action? To enable the passage (navigation) of vessels without obstruction