As Capt. Lorenzo Sitgreaves set out in 1851 to explore the southern portion of the Four Corners region (won in the recent war with Mexico), his party included Dr. Samuel Woodhouse, a thirty-year-old physician and naturalist who kept a journal of their travels from New York to California. Woodhouse recorded three weeks in San Antonio, made daily entries across the Trans-Pecos, and, after a hiatus in Santa Fe, resumed his journal on the march to Zui Pueblo. Midway into their three weeks at Zui, he nearly died from a rattlesnake bite and was scarcely recovered when the explorers again started west. The largest part of Woodhouses journal concerns Captain Sitgreaves reconnaissance for a wagon road westward from Zui to the Colorado River of the West. It also records a perilous, starving descent of that untamed river to the Yuma Crossing. The doctors entries grew with scientific curiosity and increasing concern for finding water and meeting hostile natives. His extensive notes on plants and animals were part of the first effort to describe and map what is now northern Arizona. His diaries also provide the first detailed description of the Walapai and Mohave peoples the explorers encountered. Sam Woodhouse’s private journal is published here for the first time. Although the basic facts of the Sitgreaves expedition have long been known, the journal adds much detail and great depth to the story, allowing the editors to draw credible conclusions about natural science and Southwestern exploration in the mid-nineteenth century. The color plates reproduce some of the earliest chromolithography done in the United States.