It is formed by convergence of the Wellow Brook and Cam Brook at Midford. It passes Tucking Mill and joins the River Avon close to the Dundas Aqueduct and the remains of the Somerset Coal Canal. For its entire length it defines the boundary between Bath & North East Somerset and Wiltshire.
Although the Midford Brook is named on Ordnance Survey maps, the Environment Agency does not recognise it, instead identifying the Wellow Brook as continuing to the Avon.
The Midford Brook has a catchment area of 147.4 km2 which is largely over impermeable Lias. The deep steep-sided valley means that it responds rapidly to rainfall.