Final answer:
The Mississippi River formed the western boundary of the Northwest Territory, serving as a natural geographic feature that defined political borders.
Step-by-step explanation:
The physical feature that formed most of the western boundary of the Northwest Territory is the Mississippi River. Natural physical geographic features often determine political boundaries, and this is clearly seen in the establishment of the territory's boundary. The Mississippi River served as a natural border, separating the Northwest Territory from areas to the west, particularly Spanish Louisiana. Additionally, the Ohio River played a significant role in shaping the region's dynamics, particularly in terms of the sectional struggle regarding slavery, with free states north of the river and slave states to the south.