Complex sentence: The headwaters of the East Branch North Fork and Middle Fork originate in the Diamond Mountains and flow west through the Sierras, and all the fork/branch tributaries of Lake Oroville discharge into arms of the reservoir in the foothills northeast of Oroville in eastern Butte County.
Split into simpler sentences: The headwaters of the East Branch North Fork and Middle Fork originate in the Diamond Mountains and flow west through the Sierras. The four forks unite as arms of the Lake Oroville reservoir in the foothills northeast of Oroville in eastern Butte County.