The Wylye rises below the White Sheet Downs just south of Maiden Bradley in the west of Wiltshire, then flows north through the Deverill valley. A tributary which feeds the man-made Shearwater lake joins near Crockerton. On the southern edge of Warminster the river turns to head generally east south east, forming the Wylye Valley, into which the A36 road and the Wessex Main Line are also squeezed. The river passes through the parishes of Norton Bavant, Heytesbury, Knook, Upton Lovell, Boyton, Codford, Wylye and Wilton, near the southern edge of Salisbury Plain. It is fed by ephemeral, winterbourne streams so water flow can vary.