A 14C age estimate of 13 470 ± 130 BP (ISGS-1378) from organic material at the base of transgressive lake deposits exposed in a southern Lake Michigan shore bluff near Riverside, Michigan, confirms that an intra-Glenwood low-water phase occurred in the Lake Michigan basin during the Mackinaw (Cary – Port Huron) Interstade. The altitude of organic material at Riverside suggests that the water plane was at or below modern lake level during the intra-Glenwood low-water phase. This observation indicates that drainage from the Lake Michigan basin was eastward, probably through the Straits of Mackinac into the Lake Huron basin. Such a drainage pattern implies that the correlative lake occupying the Lake Huron basin (Arkona low-water phase) must have had a level either congruent with or below that of the Lake Michigan basin. This lake system ultimately drained from the Lake Huron basin through a low eastern outlet (probably the Trent River lowland). The existence of low-level lakes at this time also indicates that the ice margin of the Lake Michigan lobe must have retreated at least as far north as the Straits of Mackinac region during the Mackinaw Interstade.