First-order modeling suggests that a low-water phase in late-glacial Lake Agassiz can be explained through changes in the balance between evaporation, precipitation, and runoff, rather than drainage. The low-water Moorhead Phase is often attributed to drainage through outlets opened by isostatic depression and retreat of the Laurentide ice margin. However, new data indicate that the proposed outlets were ice-covered during the Moorhead Phase. Instead, the lake water levels dropped to the Moorhead Phase before the start of the Younger Dryas chronozone and remained there until 11.3 ka. Thus, drainage seems to be an implausible explanation for Younger Dryas-aged low water levels in Lake Agassiz. An alternative explanation is that evaporation equaled or exceeded water inputs from the adjacent ice margin and the deglaciated parts of the drainage basin. To evaluate whether this hypothesis is plausible, we constructed a simple model that considers the paleo-basin geometry, hydrology, and meltwater production from the adjacent ice margin. Modest hydrologic changes (within the range of present-day variability), coupled with low meltwater production, produce a closed basin. Shifts in the location of the polar jet, driven by increased Arctic albedo, may explain our inferred hydrologic changes.