Between the acquisition of the North West Company in 1821 and the Oregon Treaty of 1846, the HBC greatly expanded the operations of the Columbia Department. The fur trade was extended to essentially every major river from the Yukon River in the north to the mouth of the Colorado River in the south, and east to the headwaters of the Missouri River tributaries. American fur trade competition was effectively blocked through various strategies, including selectively overhunting frontier regions to create "fur deserts", and the construction of forts on the Pacific Northwest coast to intercept furs before American ships could acquire them.