The River Parrett has its source in the Thorney Mills springs in the hills around Chedington in Dorset in England and flows west through the Somerset Levels. The mouth is a Nature Reserve at Burnham on Sea where it flows into Bridgwater Bay on the Bristol Channel. The river is tidal for 18.6 miles (30 km) up to Oath; and, because the fall of the river, between Langport and Bridgwater is only 1 foot per mile, or 20cm per km, it is prone to frequent flooding, in winter and high tides.
The River Parrett is 37 miles (60km) long and its main tributaries include the Rivers Tone, Isle and Yeo. The River Cary drains into the Parrett via the King's Sedgemoor Drain. The River Parrett drains an area of over 1,690 km2 (652.5 sq mi), comprising around 50% of the land area of Somerset.