In addition to Dichotomy, it’s important to mention the piece that made Riegger’s name, Study in Sonority. He wrote it in 1927 for his Ithaca Conservatory summer-term orchestra; the group had mostly violins, so he wrote for 10 violinists. At Eastman, Howard Hanson programmed the work before Stokowski got a hold of it and performed it with the Philadelphia Orchestra strings in 1929 at Carnegie Hall and in their home city.
The New York Philharmonic played the piece a couple of times in the 1960s, and in 1994 at the American Eccentrics festival. There’s nary a commercial recording to be had, but you can view the score on the New York Philharmonic’s digital archives.