The Exeter Ship Canal, sometimes just called the Exeter Canal, downstream of Exeter, Devon, England was built in the 1560s which means it pre-dates the "canal mania" period and is one of the oldest artificial waterways in the UK.
At the start of Exeter's history, The River Exe was tidal and navigable up to the city walls enabling it to be a busy port. In the 1270s or 80s, the Countess of Devon, Isabella de Fortibus, built a weir across the river to power her mills (this weir is remembered in the name of the nearby suburb Countess Wear). This had the effect of cutting off Exeter's port from the sea and damaging its salmon fisheries.
In 1290, trade with Exeter's port was restored, only to be blocked by a new weir built in 1317 by Hugh de Courtenay, 9th Earl of Devon (Isabella's cousin), who also built a quay at Topsham. Because of the blockages on the river, boats were forced to unload at Topsham and the earls were able to exact large tolls to transport goods to Exeter.