While exploring the Louisiana Purchase with Lewis and Clark, he took ill at the end of July 1804. On July 31, Floyd wrote in his diary, "I am very sick and have been for sometime but have recovered my health again." However, this apparent recovery was soon followed by a severe turn for 
the worse. William Clark described Floyd's death as one "with a great deal of composure" and that before Floyd died he said to Clark, "I am going away. I want you to write me a letter."
A funeral was held and Floyd was buried on a bluff overlooking the Missouri River. The expedition named the location Floyd's Bluff in his honor. They camped that night at the mouth of Floyd River (which was named for him), "about 30 yards wide, a beautiful evening.--"
Clark diagnosed the condition which led to Floyd's demise as bilious colic, though modern doctors and historians believe Floyd's death was more likely to have been caused by a ruptured appendix. The brief "recovery" Floyd described may have represented the temporary relief afforded by the bursting of the organ, which would have been followed by a fatal peritonitis. If that were the case, because there was no known cure for appendicitis at that time, he would have been no better off had he been with the best physicians of the day.
The Sergeant Floyd Monument, Sioux City, Iowa is seen here
