Just past Tuckup Canyon, at mile 165, the south edge of the Colorado River forms the boundary of the Hualapai Reservation. A few miles downstream on the right, 3,000-foot high cliffs loom above the river. On top is Toroweap, a remote overlook on the North Rim’s western reaches.

Below, at mile 179, is Lava Falls. Some consider these falls, a class 10 rapid, the fiercest white water in Grand Canyon.

Here, the Colorado River drops 37 feet in just a couple of hundred yards. In 1869, John Wesley Powell’s men chose to make an arduous three-hour portage, rather than risk their wooden boats and remaining food stores against the churning rapid.

Powell recognized the signs of past volcanic activity along this section of the river. Upriver from the rapid, a volcanic neck, the black monolith Vulcans Forge, juts out of the water. Downriver, cascades of basalt rock mark the canyon walls, the flow of a volcano that erupted 1.2 million years ago. The flows created a dam about 1,400 feet high. An even higher dam formed near Prospect Canyon, creating a lake that extended all the way to present-day Moab.

Lava flows are also evident at Whitmore Wash, at mile 188. The canyon walls are lower here, and the trail that leads up to the rim here is only a mile, the shortest in the canyon. (The Lava Falls Trail, at 1.5 miles, is the shortest rim-to-river trail entirely within the bounds of the park.)