The Fortescue River river rises in the Ophthalmia Range and enters the Indian Ocean on current day Mardie Station, about 75 kilometres south-west of Dampier.

Approximately 760 km in length, it is Western Australia’s third longest river.

It was named by surveyor/explorer F.T.Gregory, leader of the North-West Australian Exploring Expedition in 1861. Gregory recorded that he named the river after:

“the Under Secretary of State for the Colonies, under whose auspices the expedition took its origin.”

This was Chichester Samuel Fortescue, 2nd Baron Clermont and 1st Baron Carlingford KP, PC (18 January 1823 – 30 January 1898), known as Chichester Fortescue until 1863 and as Chichester Parkinson-Fortescue between 1863 and 1874 and Lord Carlingford after 1874,  a British Liberal politician of the 19th century. He was Under Secretary from 1857 to 1865.

Gregory also named the nearby Chichester Range after Fortescue during the same expedition.

Sources:

Geonoma
F.T. Gregory’s journal, ‘Explorers’ Diaries’, vol. 5, p.104.
Office-Holders in Modern Britain: Volume VI: Colonial Office Officials 1794-1870, published 1976