[2] Its namesake was Captain James Johnson Southgate, a retired ship-master, who came to Victoria in 1859 via San Francisco and launched a commission and general mercantile business, largely in connection with the Pacific Station of the Royal Navy at Esquimalt, operating as J.J. Southgate & Co.

The river is approximately 65 kilometres (40 mi) in length, beginning on the western flank of Good Hope Mountain, to the east of the Homathko Icefield, and then flows generally south-southwest for about 40 kilometres (25 mi) before turning west-northwest toward the head of Bute Inlet.

On 28 November 2020, unseasonably heavy rainfall triggered a landslide of 18,000,000 m3 (24,000,000 cu yd) into a glacial lake at the head of Elliot Creek.

The sudden displacement of water generated a 100 m (330 ft) high megatsunami that cascaded down Elliot Creek and the Southgate River to the head of Bute Inlet, covering a total distance of over 60 km (37 mi).

The event generated a magnitude 5.0 earthquake and destroyed over 8.5 km (5.3 mi) of salmon habitat along Elliot Creek.