Complex sentence: British explorer David Thompson (explorer), the first European to navigate the entire length of the Columbia River in 1811, reached the partially constructed Fort Astoria at the mouth of the Columbia, arriving two months after the Pacific Fur Company's ship, the Tonquin.
Split into simpler sentences: British explorer David Thompson (explorer), in 1811 navigated the entire length of the Columbia River. He reached the partially constructed Fort Astoria at the mouth of the Columbia, two months after the Pacific Fur Company's ship, the Tonquin.