What are the motifs in The Exploration of the Colorado River and Its Canyons by John Wesley Powell?

Danger is a recurring idea. The members of the expedition were basically sailing into the unknown. There was no way for them to know what to expect. They had to constantly be on the alert for dangers in the water. They had many experiences with rough rapids, eddies and whirlpools. Many times their boats capsized and their oars were lost. Even one of the boats was destroyed from smashing against the rocks in the rough waters. They lost many of their supplies in this way and had to frequently stop to make new oars and to repair damages. When they came upon cataracts and falls, they had to portage their boats and supplies.