Complex sentence: The Nez Perce territory at the time of Lewis and Clark (1804--1806) was approximately and covered parts of present-day Washington, Oregon, Montana, and Idaho, in an area surrounding the Snake, Salmon and the Clearwater rivers.
Split into simpler sentences: The Nez Perce territory at the time of Lewis and Clark (1804-1806) was approximately. It covered parts of present-day Washington, Oregon, Montana, and Idaho, in an area surrounding the Snake, Salmon and the Clearwater rivers.