A treaty with the Winnebagoes, concluded at Washington City, was proclaimed, by which the Winnebago Nation ceded all their land east of the Mississippi River.

The evidence

• “Einally, and to remove the possibility of any doubt whatever as to the validity of the title held by the United States to the lands lying on both sides of Rock River, a treaty was concluded at Washington City — proclaimed June 16, 1838 — with the Winnebagoes, whereby, in brief but comprehensive terms, " the Winnebago Nation of Indians cede to the United States all their land east of the Mississippi River."”

  1879-history-of-rock-county/08-rock-county-ch02-mounds-indians-black-hawk-settlement.md, PDF p. 324 / book p. 328

  Butterfield, C. W., et al. The History of Rock County, Wisconsin: its early settlement, growth, development, resources, etc. Chicago: Western Historical Company, 1879. Cornell University Library via the Internet Archive, archive.org/details/cu31924028871585.

  Grade A: Found on a page of one of our sources