19th century
- 1805 - October 16: The Lewis and Clark Expedition passes through the Tri-Cities area heading west, camping near present-day Sacajawea State Park and visiting Bateman Island.[2]
- 1806 - April 28: The Lewis and Clark Expedition passes through the Tri-Cities area on their return journey.[3]
- 1811 - Members of the Astor Expedition likely passed through the Tri-Cities traveling on the Snake and Columbia Rivers.[4]
- 1824 - June 29: As part of the Oregon boundary dispute, the British propose a boundary between the United States and British North America using the Columbia River in a manner that would have split the region, placing what is now Kennewick and Richland in British territory with the Pasco side of the river being American territory. This proposal was rejected by the United States.[5]
- 1847 - The St. Rose of Chemna Catholic mission was established approximately 3 miles from the confluence of the Columbia and Yakima Rivers on the south bank, at the approximate location of the current I-182 bridge in South Richland. The mission was reported abandoned as late as 1859.[6][7][8]
- 1855 - The Umatilla and Yakama tribes cede the land the Tri-Cities sit on at the Walla Walla Council.[9]
- 1879 - Ainsworth becomes the first community platted. The town, which no longer exists, was in what is now the east end of Pasco.[10]
- 1883 - November 28: Franklin County splits from Whitman County with Ainsworth as the county seat.[11][12]
- 1884 - The Oregon Steam Navigation Company opens a railroad bridge over the Snake River at Ainsworth.[12]
- 1885 - The seat of Franklin County moves from Ainsworth to Pasco.[12]
- 1888 - The Northern Pacific Railroad opens a railroad bridge across the Columbia River.[13]
- 1891 - September 3: Pasco is incorporated.[14]
- 1894 - February 1: Weather records begin to be taken at Kennewick.[15]
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