In August 1811, three months after Fort Astor was established, David Thompson and his team of British North West Company explorers came floating down the Columbia to Fort Astoria. He had just completed a journey through much of western Canada and most of the Columbia River drainage system. He was mapping the country for possible fur trading posts. Along the way he camped at the confluence of the Columbia and Snake rivers and posted a notice claiming the land for Britain and stating the intention of the North West Company to build a fort on the site (Fort Nez Perces was later established there). Astor, concerned the British navy would seize their forts and supplies in the War of 1812, sold to the North West Company in 1812 their forts, supplies and furs on the Columbia and Snake River. The North West Company started establishing more forts and trading posts of its own.