J Harlen Bretz[1] (September 2, 1882 – February 3, 1981) was an American geologist, best known for his research that led to the acceptance of the Missoula Floods.

He made important discoveries regarding the origin of the Channeled Scablands and wrote an extremely influential paper on the morphology and origin of limestone caves together with a guide to the caves of Illinois. He wrote a detailed study of Missouri caves published as The Caves of Missouri in 1956.

Bretz started his career as a high school biology teacher in Seattle. During this time he began studying the glacial geology of the Puget Sound area, and eventually went to the University of Chicago where he earned his Ph.D. in geology in 1913. He became an assistant professor of geology, first at the University of Washington and then the University of Chicago.