Final answer:
From 1810 to 1820, the key characteristic of the non-Indian population west of the Appalachian Mountains was agricultural expansion, marking a phase of settlement and initial economic development in the region.
Step-by-step explanation:
Between 1810 and 1820, the non-Indian population west of the Appalachian Mountains was primarily characterized by agricultural expansion. As Americans moved into the territory of the Old Northwest and the Old Southwest, they claimed land and began to cultivate it, contributing to the raw materials needed for the burgeoning number of factories in the Northeast.
While individual opportunity spurred on the early settlers, over time, larger economic forces such as big business began to take hold and solidify the American way of life as envisioned by eastern proponents of Manifest Destiny.