A Swiss-born park designer and conservationist, Riis was the original superintendent of the Rockford, Illinois, park system, from 1912 to 1927. He was the director of the American Institute of Park Executives from 1923 to 1926, and he served as the editor and primary writer of the “Conservation of Wild Life” section of Parks and Recreation Magazine for 16 years. From 1927 to 1932, Riis was instrumental in the design of North Park and South Park in Allegheny County, Pennsylvania, the state's first two regional parks outside Pittsburgh. His work was influenced by that of another Midwestern designer, Jens Jensen. Like Jensen, Riis promoted the use of native species and local, natural materials to construct such landscape features as stone swimming pools, council rings, waterfalls, and shelters. Dismissed by the Park Commission in 1932, he returned to the Midwest where he focused on work for private clients, perhaps most notably the estate of Howard Colman in Rockford, Illinois. Riis also conducted pioneering deer-browse studies with conservationist Aldo Leopold.