Cabeza de VacaCabeza de Vaca, an explorer of the Americas in the 1500s, wrote the first book about the land that is now Texas. Historians have vigorously debated whether he followed a northern route across Texas or a southern route that crossed the Rio Grande and spent much time traveling west in Mexico. Cabeza de VacaCabeza de Vaca provided an important clue with his description of crossing a river as wide as the one in SevilleSeville, Spain. Various scholars proposing route theories have identified this river as the Trinity, the Frio, the Colorado, the Guadalupe, the Concho, or the Rio Grande. Can we show that all previous scholars made a large mistake (a factor of two) regarding the width of the river in SevilleSeville, Spain? What new evidence allows us to correct the errors in the previous literature? What sixteenth-century books about SevilleSeville, Spain include descriptions of the width of the river in that city? What is the correct interpretation of a measurement carried out in SevilleSeville, Spain by a scholar in the nineteenth century? How do entries in nineteenth-century dictionaries help in resolving this controversy? Which river did Cabeza de VacaCabeza de Vaca cross in his exploration? Do the clues, correctly interpreted, support the trans-Texas northern route or the southern route across the Rio Grande into Mexico?