In 1869, the Colorado River was considered a geologic treasure. Still unmapped, it was “the last unexplored territory in the United States.” John Wesley Powell changed that with his expedition, recounted in this latest installment in the Great Explorers series. Powell was 5’6” tall, weighed 120 pounds and only had one arm, after a severe wound at the Battle of Shiloh in 1862. He had never piloted a boat through rapids, but he was determined to explore for Colorado, gather scientific information and have an adventure at the same time. The dense text is balanced by dramatic illustrations and many sidebars with excerpts from the journals of Powell’s men. Especially useful is the map that opens on a gatefold and can stay opened to help readers follow Powell’s adventures down the Colorado River. Though Waldman states that this was, first and foremost, a scientific expedition, the volume emphasizes the drama as well. The appealing illustrations will lure readers, who will be caught up in this true, but little-known tale. (afterword, sources, index) (Nonfiction. 9+)