Marsh here refers to the two other men named Davenport who were at Rock Island that summer.
George Davenport: Col. George Davenport (1783-1845) emigrated to the U.S. in 1804 and arrived at Rock Island in 1816 with the troops who built Ft. Armstrong. He became successively a fur trader, Indian agent, ferry operator, postmaster, and real estate speculator. When Marsh met him in 1836, he was both the wealthiest and most influential person for hundreds of miles. He was trusted and admired by the Fox Indians, who performed their own funeral rites for him when he was murdered in 1845. Davenport, Iowa, is named for him.
Col. William Davenport ("commanding officer"): William Davenport was born in Pennsylvania, joined the Army during the War of 1812, and was appointed a colonel in 1832. He served as commander of Fort Armstrong on Rock Island from June 1833 until 1836, when the fort was closed and he moved with the garison to Fort Snelling, at modern St. Paul, Minn. In 1841-42 he was commander of Fort Crawford in Prairie du Chien. He resigned from the Army in 1850 and died in 11858.
These two joined the Indian agent Marmaduke Davenport, whom Marsh had met two days previously. Oddly enough, the three Davenports appear to have been entirely unrelated.
Fort Armstrong, Rock Island: The fort commanded by Col. Davenport was built in 1816 on Rock Island to control the Upper Mississippi region; it was closed in 1836 and the garrison moved upriver to Fort Snelling. A year after Marsh visited, George Catlin painted it. See the U.S. Army Web site for more information about the history of the fort.