Complex sentence: Exploration by Lewis and Clark (1805--1806) and the United Kingdom's David Thompson, who extensively explored the Columbia River from 1807-1811, publicized the abundance of fur-bearing animals in the area.
Split into simpler sentences: Exploration by Lewis and Clark (1805--1806). The United Kingdom's David Thompson, extensively explored the Columbia River from 1807-1811, and publicized the abundance of fur-bearing animals in the area.