There are 20 US Post Offices in Yakima County serving a total of 162,569 residents equating to 8,128 residents per post office. It's estimated that approximately 259,478 packages are handled across the 20 county post offices annually. Yakima County is a county in the U.S. state of Washington. As of the 2010 census, its population was 243,231. The county seat and largest city is Yakima. The county was formed out of Ferguson County on January 21, 1865 and is named for Yakama tribe of Native Americans. Ferguson County, no longer in existence, had been created from Walla Walla County on January 23, 1863. Yakima County comprises the Yakima, WA Metropolitan Statistical Area. The Yakama Indian Reservation, the 15th largest reservation in America, covers 1,573 mi\u00b2 (4,074 km\u00b2), comprising 36% of the county's total area. Its population was 31,799 in 2000, and its largest city is Toppenish."