Complex sentence: Originally known as the Pole River, the lower Birkenhead's valley is part of the Long Portage of the Douglas Road, also known as the Pemberton Pass (officially), Birken Pass, Gates Pass, and (in gold rush times) Mosquito Pass.
Split into simpler sentences: Originally known as the Pole River, the lower Birkenhead's valley is part of the Long Portage of the Douglas Road, also known as the Pemberton Portage. The height of land on this route has been variously called Pemberton Pass (officially), Birken Pass, Gates Pass, and (in gold rush times) Mosquito Pass.