The river is named after John James Gregg (1840–1941), a prospector and trapper prominent in the area.

[1] The Gregg River forms at the confluence of a number of minor creeks near the Cardinal River Coal Mine, at the base of Mount Sir Harold Mitchell.

The river then flows northwest, taking on a number of tributary creeks before joining the McLeod River, which in turn flows into the Athabasca River.

The Gregg is bridged by Alberta Highway 40.

This article related to a river in Alberta, Canada is a stub.