The fort felt into decline sometime after the French and Indian War (1754–1763). During this time, the area in which Fort de Cavagnial came under the power of Spain, although French soldiers remained at the site of the fort until 1764, when Pierre-Joseph Neyon de Villiers (acting on behalf of Jean-Jacques Blaise d'Abbadie, the director-general of the Colony of Louisiana) ordered that the troops stationed in the area to "throw down" the outpost and retreat to New Orleans. Evidently, the site was not completely razed, for in 1804, Meriwether Lewis and William Clark stumbled across the old site during their westward expedition, noting in their journal that "[Fort de Cavagnial] may be recognised by some remains of chimneys, and the general outline of the fortification, as well as by the fine spring which supplied it with water." In 1819, explorers on Stephen Harriman Long's Yellowstone expedition also made note of the ruin, describing them as "a few miles below Isle au Vache." By the mid-19th century, all traces of the fort were gone.