South of the coastal plain, the mountains of the eastern Brooks Range rise to nearly . This northernmost extension of the Rocky Mountains marks the continental divide, with north-flowing rivers emptying into the Arctic Ocean and south-flowing rivers joining the great Yukon River. The rugged mountains of the Brooks Range are incised by deep river valleys creating a range of elevations and aspects that support a variety of low tundra vegetation, dense shrubs, rare groves of poplar trees on the north side and spruce on the south. During summer, peregrine falcons, gyrfalcons, and golden eagles build nests on cliffs. Harlequin ducks and red-breasted mergansers are seen on swift-flowing rivers. Dall sheep, muskoxen, and Alaskan Arctic tundra wolves are active all year, while grizzly bears and Arctic ground squirrels are frequently seen during summer but hibernate in winter.