The river is formed by water entering swallets in the limestone and rises from the ground at Wookey Hole Caves in the Mendip Hills in Somerset, and runs through a V-shaped valley.

[5] It then passes through major cave systems such as Swildon's Hole, Eastwater Cavern and St Cuthbert's Swallet, around Priddy,[6][7] but 95% is water that has percolated directly into the limestone.

From this point until it passes between Uphill Cliff and Brean Down, then reaches the coast at Weston Bay, the river demarcates Sedgemoor from North Somerset.

The name derives from a Common Brittonic word meaning "abounding in fish", which is also the root for the River Axe in Lyme Bay as well as the Exe, Esk, Usk and other variants.

In the 14th century a French ship sailed up the river and by 1388 Thomas Tanner from Wells used Rackley to export cloth and corn to Portugal, and received iron and salt in exchange.