Complex sentence: The main stem Feather River begins at the outlet of Oroville Dam's hydroelectric plant, which forms Lake Oroville, a reservoir where its four long tributary forks join together--the South Fork, Middle Fork, North Fork, and West Branch Feather Rivers.
Split into simpler sentences: The main stem Feather River begins at the outlet of Oroville Dam's hydroelectric plant. The Oroville Dam forms Lake Oroville, a reservoir where its four long tributary forks join together--the South Fork, Middle Fork, North Fork, and West Branch Feather Rivers.