What natural landmark formed the new western boundary of the United States?
The whole history of the US has been shaped by the fact that there was no natural boundary of the US until national expansion reached the Pacific in the 1840s.
One of the biggest issues the colonists had against the British was the imposition of an artificial western boundary. Lawmakers in London had tried to prohibit the colonies from expanding past the high ridges of the Appalachian Mountain chain, because they feared the colonies would be difficult to rule if they spread out too far from the coast.
When the US negotiated its peace treaty with Great Britain in 1783, the western boundary of the US, in most places, was the Mississippi River. Land west of there belonged to France and Spain.
President Jefferson bought Louisiana from France in 1803, which pushed the boundary further west to the Spanish lands in Texas in the south and the Rocky Mountains farther north. The Lewis and Clark Expedition gave the US a claim to the Oregon Territory (which included the modern day states of Oregon, Washington and parts of Idaho and Montana) that competed with Great Britain's claim to the area through their rule of Canada.
When Mexico won its independence, the Spanish lands in Texas, Nevada, California, etc. became Mexican. American farmers and ranchers had already become the predominant population in Texas and in 1836 they broke away from Mexico. In 1845, the Republic of Texas applied for -- and received -- statehood.
Mexico was not pleased by that action and the dispute resulted in war with Mexico in 1846. At the same time, the US concluded a treaty with Great Britain in 1846 that set the boundary with Canada all the way to the Pacific and gave Oregon to the US.
Victory against Mexico in 1848 concluded the American push all the way to the Pacific through the peace treaty.