Which river is the longest in the British Isles?

The River Shannon is the longest river in the British Isles. It runs for about 360 kilometres from Shannon Pot in County Cavan through central and western Ireland before reaching the Atlantic through the Shannon Estuary.

The Shannon has been an important route since antiquity. The Greek-Egyptian geographer Ptolemy included it on maps in the second century AD. Today, its basin drains roughly one-fifth of Ireland, and the river forms a major geographical divide between much of the west and east of the island.

A common mix-up is the River Severn, which is the longest river in Great Britain, but not in the British Isles as a whole. The British Isles includes Ireland, making the Shannon the standard answer. Exact measurements vary because the river flows into a long tidal estuary, but those differences do not change the quiz answer.