Complex sentence: Constricted by two steep subvertical granodiorite walls, the incredibly narrow passage and high water velocity made this part of the upstream journey extraordinarily difficult for salmon, and they would hover along the shores of the river or rest in its back-eddies.
Split into simpler sentences: Constricted by two steep subvertical granodiorite walls, the incredibly narrow passage and high water velocity made this part of the upstream journey by salmon extraordinarily difficult. Salmon would closely hover along the shores of the river or rest in its back-eddies.