Complex sentence: The name "Grand" refers to the historical "Grand River", which was renamed the upper Colorado River in 1921, and the word "Junction" is from the confluence of the Colorado and Gunnison rivers.
Split into simpler sentences: The name "Grand" refers to the historical "Grand River;" it was renamed as the Upper Colorado River in 1921. The word "Junction" refers to the confluence of the Colorado and Gunnison rivers.